12
MAINLY PHYSICS
CONTENTS of this section:
12.1
12.2
12.2.1
12.2.2
12.2.3
12.2.4
12.2.5
12.2.6
12.3
12.3.1
12.3.2
12.3.3
12.3.4
12.4
12.4.1
12.4.2
12.4.3
12.4.4
12.4.5
12.4.6
12.4.7
12.5
12.6
12.7
12.7.1
12.7.2
12.8
12.8.1
12.8.2
12.8.3
12.9
12.9.1
12.9.2
12.9.3
12.9.4
12.9.5
12.9.6
Page
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1202
Alternative apparatus systems compared 1202
Bulbs and LEDs
1203
Cells or power packs?
1204
Meters for circuit work
1204
Maintenance
1205
Digital multimeters
1206
DMMs compared to analogue meters 1207
Provision of digital multimeters
1208
Which DMMs to buy
1208
New uses
1209
Dynamics
1210
Introduction
1210
Timing methods
1210
The air track
1216
Frictionless tables
1216
Trolleys and runways
1217
Other dynamics equipment
1218
Other means of recording linear motion 1219
Electrolytic capacitors
1219
Electron-beam and similar tubes
1220
Electronic meters
1221
The different meters described
1221
Experiments using the meters
1224
Heat experiments
1227
Calorimetry
1227
Cooling curves
1228
Gas laws
1228
High voltages
1229
Electrostatic generators
1230
EHT units
1232
HT units
1232
Induction coils
1232
Demountable transformers
1232
Power line (National Grid) simulation 1232
12.1
Beams and rings for lifting
Beams and rings for lifting
Circuit work
Introduction
12.9.7
Household power circuit simulation
12.10
Ionising radiations
12.10.1
Category C work
12.10.2
More details on Category C work
12.10.3
Category B work with open sources
12.10.4
Electron beam tubes etc
12.10.5
Apparatus for work with radioactive
substances
12.10.6
Units and definitions
12.11
Kinetic theory models
12.12
Lasers
12.12.1
Use of lasers
12.13
Mercury
12.13.1
Situations where care is needed
12.13.2
Precautions when handling mercury
12.14
Oscilloscopes
12.14.1
Use
12.14.2
To teach ‘the Oscilloscope’ & ‘the CRO’
12.14.3
Use with VELA
12.14.4
Explanation of terms and features
12.14.5
What to buy
12.15
Pulleys and hoists
12.16
Ray sets
12.17
Ripple tanks
12.17.1
Features, accessories and their use
12.17.2
Use of ripple tanks
12.18
Stretched wires etc
12.19
Stroboscopes
12.20
Vacuum
12.21
Wave machines
12.22
Magnets
12.22.1
Precautions
12.22.2
Magnets in science
12.22.3
Care
12.22.4
Re-magnetising
12.22.5
Background information
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Some laboratories are fitted with a beam close to the ceiling to which apparatus can be
attached. An alternative is a hook or a ring (a ‘point loading hook’) attached to a beam
within the ceiling. While it is unlikely that the beam or ring could be inadequate for
the loads attached to it during normal school science, it should be labelled with the
maximum load that can safely be attached.
Maximum load
This will have been specified before the design of the laboratory and checked by
the contractor’s structural engineer. If the maximum load a beam can support is
unknown and cannot be found on the plans of the building, a structural engineer
would need to be consulted.
Inspection
Unless a school uses a beam frequently for loads approaching its maximum limit,
safety inspections are not needed.
If hoists attached to such beams are used to lift pupils or heavy objects, great care
must be taken with the attachments and the condition of the hoist etc. See section
12.15 (Pulleys and hoists).
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1202
12.2
Circuit work
12.2.1
Introduction
© CLEAPSS 1992
Work with electric circuits is an essential part of the National Curriculum, beginning
with simple series work at KS1 and KS2 to more advanced work beyond. It can be
dogged with problems of excessive use of consumables (disposable cells and bulbs) and
of poor contacts. In secondary schools, there is a strong argument for a fresh approach
with different apparatus. The cost of consumables can be reduced or eliminated by
using LV power units instead of cells and light-emitting diodes instead of torch bulbs.
These solutions are considered below but have some educational disadvantages.
This section starts by considering which equipment to use but there are no simple
answers and all makes of circuit boards and circuit kits require some maintenance.
Suggestions for maintenance are given in section 12.2.6.
Details of circuit kits and boards available commercially are given in the CLEAPSS
guide R122s, Circuit work for secondary schools.
12.2.2
Alternative apparatus systems compared
Table 12.1 compares the several systems of apparatus which can be used.
Table 12.1
A brief comparison of different circuit systems
System
Features
Note
Worcester Circuit
Board, from Griffin
& George, Philip
Harris, Loctronics
etc
Mounted components fitted between metal pillars on a Gives very clear circuits which can be altered
board. Torch bulbs are used as current indicators.
rapidly. These properties were claimed to lead to
better understanding. The system has been criticised for problems caused by poor maintenance. It
is now rather expensive.
4 mm systems, eg,
Unilab
Mounted components with 4 mm sockets connected Gives fairly clear circuits which can be altered
with leads terminating in 4 mm plugs. Torch bulbs are rapidly. These are more like ‘real’ circuits than
those on Worcester Circuit Boards.
used as current indicators.
Other connection
systems
Mounted components with leads connected using Gives fairly clear circuits which can be altered but
crocodile clips, springs etc. Torch bulbs are used as less rapidly. These systems are usually cheaper
than the above but contacts are less reliable.
current indicators.
Low current
systems
Mounted components with leads connected by a A major criticism of the use of LEDs is that the
variety of methods. The main feature is the use of low current path is not so clear.
current indicators such as low-current bulbs to
conserve batteries or LEDs to conserve batteries and
bulbs (see 12.2.3 below).
Detailed comparison of boardless systems and Worcester Circuit
Boards
Advantages of
the Worcester
Circuit Board
a)
There is no tangle of wires so that the circuit can be seen clearly. It can easily
be made to match the circuit diagram, important for learners at Key Stage 3.
b)
Changes to a circuit, eg, moving the position of a switch, are usually easy to
make and always clear to see.
c)
Power is usually provided by one, two or three dry cells mounted on the
board. The number connected into a circuit is easy to see and to alter, making
possible a simple introduction to electromotive force.
d)
Less able pupils can be helped with circuits drawn on duplicated paper
templates fitted over the pillars.
© CLEAPSS 1992
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e)
Cells and some other components, eg, connectors and lamps, can be kept on
the board, so easing distribution.
f)
With this arrangement, it is easy to check that all components are returned.
g)
It is likely to be a change for pupils from primary school equipment!
Disadvantages of a) Circuits on it are wired differently from how circuits are usually wired where
practical circuits do not so obviously match the theoretical circuit.
the Worcester
Circuit Board
Circuit diagram
and circuit board
12.2.3
As usually wired with single-cored leads
b)
The layout of circuits on a Worcester Circuit Board stresses the relative
position of components, not the connections to them. For example, it is
difficult to illustrate the two wired circuits in the diagram above on a board.
c)
Contact resistances between connectors of all patterns and the pillars can be
1-2 × 10-1 Ω or higher still if the kit is not well maintained; this can lead to
problems. 4 mm plug-socket contact resistances are usually lower.
d)
A board is expensive.
e)
Leads fitted with 4 mm plugs and crocodile clips are needed for meters and
other extra components so the system is a hybrid one.
f)
It is not easy to use its components for other purposes as they cannot be used
directly with 4 mm leads.
Bulbs and LEDs
Bulbs: which
value to use
Originally 1.25 V 0.25 A bulbs were specified for the Worcester Circuit Board. It
was considered educationally valuable for pupils to blow one or two, which, of
course, happens after a few seconds if a bulb of this type is connected across three
cells. In practice, replacing bulbs and maintaining a stock of spares was regarded
as a nuisance as well as a needless expense. Further, pupils do not always discard
blown bulbs which remain to bug future work and, with holders of some models ,
replacing bulbs is not easy. Therefore, most schools now use the more robust
2.5 V 0.3 A bulb and, perhaps, fit each board with only two cells.
It is interesting to note that one kit available contains 4.5 V 0.3 A bulbs. These do
not burn out rapidly with three cells but still give an indication, a faint but quite
visible glow, with one.
Low-current
bulbs
Some circuit systems use low-current bulbs, eg, 6 V 60 mA. Usually the system is
primarily designed to teach electronics, requiring a 6 V power supply perhaps
provided by a ‘lantern’ battery. With this supply voltage, the more common lowervoltage ‘torch’ bulbs would burn out. The disadvantage with this system is that it
is impossible to power a bulb with more than one battery to show the effect of an
increase of emf in a circuit; with two batteries, a bulb would certainly show a
bigger current by being brighter but would almost instantly burn out.
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Light-emitting
diodes in place
of bulbs
The quality of
filament lamps
12.2.4
1204
© CLEAPSS 2005
LEDs are comparable in price to bulbs, have a long life, are more mechanically
robust and draw less current from the cells. However, their use for this purpose
produces a number of disadvantages.
a)
Their use is more complicated as they need a protective resistance and their
polarity must be observed.
b)
The change of brightness with current is considerably less marked compared
to bulbs.
c)
Milliammeters are needed for current measurement. The milliampere is not
so suitable for beginners as the ampere, although the avoidance of the
decimal point is an advantage, and the meters are more vulnerable to
electrical overload.
d)
The main objection to the use of LEDs is that the current path through them
is not as obvious as that through a filament lamp; also, some teachers think it
unwise to introduce semi-conductor devices before concepts of conductors and
insulators have been established.
Cheap bulbs have the disadvantage that, connected in series, they are not
uniformly bright, confusing pupils who are beginning to learn that the current in
all parts of a simple circuit is the same. Satisfactorily uniform bulbs can be
obtained from the usual suppliers, RS Components or from some local sources.
Purchasers should try a small sample of particularly cheap bulbs before buying a
large quantity.
Cells or power packs?
Advantages of
cells over LV
supplies
a)
Cells are simpler to use: one, two or three can be included in a circuit and the
effect noticed, one can be reversed etc.
b)
They are already familiar to pupils from torches and toys.
c)
The voltage is restricted to that of one, two or three cells. LV supplies without
a locking device will burn out bulbs rapidly and may cause damage to meters
(see section 12.2.5 below).
d)
The concept of emf can be introduced through cell counting (as in Nuffield
Science).
Disadvantages of a) Two or three cells per board for a class set of 16 boards is expensive.
cells over LV
b) They can be flattened easily and this can be detected only by testing.
supplies
Perhaps some compromise should be reached, using cells for particular aspects of
circuit work but LV supplies for revision of simple ideas and for magnetic and heating
effects.
Using rechargeable cells for this work is discussed in detail in section 9.5.3 (Cells,
Primary or Secondary?).
12.2.5
Meters for circuit work
Pupils are usually introduced to ammeters in elementary circuit board work, for
example, to show that the current is the same in all parts of a simple circuit or that
the current flowing through two bulbs in parallel is approximately twice that flowing
through one.
The current through the bulbs used is usually 0.2-0.3 A so that, allowing for the
possibility of three bulbs in parallel, a 1 A ammeter is the most suitable and is the one
© CLEAPSS 2005
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recommended by Nuffield and other courses. The relative advantages of analogue and
digital meters are discussed in section 10.3 (Meters). Of analogue instruments,
moving-coil meters are more satisfactory for general purposes and more readily
available; however, for this introductory work moving-iron meters are satisfactory.
Use of movingiron meters
12.2.6
Moving-iron meters are mechanically more robust than moving-coil meters and
slightly cheaper. They are electrically less robust but this does not matter when
the power supply is dry cells. Their scales are usually almost linear and their
higher internal resistance is not significant. Thus, they can be used for early
circuit work. For general information on meters, see section 10.3 (Meters).
Maintenance
Circuit boards and their components need checking after every use and regular
maintenance. It is useful to have a tester for checking cells, bulbs and connectors in
the laboratory during the lesson.
The maintenance of circuit boards is not difficult and the main problem is finding time
for it. Some of it could form a suitable punishment for petty offenders! Much of what
can be done is common sense but some advice is given below. If pillars are kept tight
and connectors the right shape (and if, of curtain wire, the right length), it should
seldom be necessary to clean either.
Tester for cells,
bulbs and
connectors
Terry clip
Cell, size C
Test cells
between
these
contacts
Components are mounted on
a board or small 'platform'.
Wiring underneath is shown:
Test
bulbs
between
these
pillars
Test connectors
between these pillars
= A pillar of similar
diameter to those on
the circuit boards
Boards
Most boards consist of a plywood board fitted with cell holders and a matrix of
pillars. These are usually of nickel-plated brass rod, 3/16" (4.75 mm) in diameter
and threaded to take 2 BA units. A few years ago, Philip Harris increased the
diameter of the pillars to 5 mm. The pillars and cell contact strips frequently work
loose and need tightening. Cleaning is needed infrequently.
Pillars etc
working loose
It is worth buying a spanner which fits the nuts on the pillars, 2 BA in many
cases. A 2 BA spanner will also fit the M5 nuts on the newer Philip Harris boards.
Sets of BA spanners can be bought from good tool shops or RS Components.
Pillars will not work loose so easily if shakeproof washers are put between the
nuts and normal washers and if a dab of Loctite Lock’n Seal or Screwlock is put on
the thread. Washers and Screwlock can be obtained from RS Components, Loctite
Lock’n Seal from hardware or car accessory shops.
Cleaning pillars
Abrasives should be used with care as their regular use will remove the plating.
Household cleaners intended for wash basins such as Jif are suitable, as are metal
cleaners such as Brasso. If there is bad corrosion, perhaps because a solution from
an electrolysis cell has been spilt on a board, fine emery cloth (Grade O) or wetand-dry paper (Grit size 600) should be used.
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© CLEAPSS 2005
Connectors
The most common patterns of connector are phosphor-bronze strips and curtain
wire fitted with eyelets. Both patterns need regular reshaping. If this is done,
connectors and pillars are self-cleaning and extra cleaning is seldom required.
Reshaping
Slight deformations of the phosphor-bronze strips can be removed with the fingers
but a small vice is useful for more severe deformations and for straightening
curtain wire connectors. The phosphor-bronze strips can be given the slight curve
they require by pressing round a 1 litre plastic beaker or something similar.
It is not usually worthwhile straightening badly deformed curtain wire
connectors: it is better to keep a reel of curtain wire from which lengths can be
cut. The lengths of this type of connector may need adjustment from time to time.
Minor adjustment can be made by screwing up or unscrewing the screw eyes,
more major shortenings by cutting off two or three turns. The lengths should be
such that the connector needs to be stretched by a millimetre to fit over the
pillars.
Cleaning
Contact resistance can be reduced by the occasional cleaning of the insides of the
screw eyes on curtain wire connectors or the insides of the slots of the phosphorbronze strips. One method of doing this is to use a short length of wooden rod,
which fits inside the eyelets or slots, moisten it with Jif, and rub it inside them. If
this is done by a technician and a drilling machine is available, there is a
considerable saving in time if the rod is rotated in the drilling machine.
The connectors should be held with a suitable jig. Fine valve-grinding compound
can be used instead of Jif.
Jig for cleaning
connectors
Wooden rod
Connector
Nails with heads removed
Block of wood
Bulb holders
12.3
Bulbs may stick in some lampholders. A few drops of a graphited easing-andpenetrating oil, WD/40, or a similar preparation usually helps to release them.
Digital multimeters
The price and robustness of digital multimeters (DMMs) is such that they are now
being used as bench meters for general pupil use in school science and electronics, not
just as test instruments replacing the traditional analogue AVOmeter.
In addition to sets of inexpensive hand-held DMMs, a school may require one or two
better-quality digital multimeters, perhaps of the bench-type, for more accurate
measurements. However, hand-held DMMs are now sufficiently accurate and reliable
for most purposes.
For a discussion of single-range digital ammeters and voltmeters more appropriate for
junior use, see section 10.3 (Meters).
© CLEAPSS 1992
12.3.1
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DMMs compared to analogue meters
Advantages
Mechanical
robustness
DMMs will stand up better to mechanical shocks which damage analogue meters.
However, some may not stand up well to the frequent and rough plugging and
unplugging of leads and the frequent replacement of fuses that school work
entails.
Correct readings DMMs are easier to read than analogue instruments. The display of digits and a
decimal point reduces considerably the likelihood of a false reading. Not all
autoranging DMMs have a display of the appropriate unit (called an annunciator);
this is essential for pupil use. There is no problem over parallax error.
High impedance
The voltmeter ranges typically have resistances of 10 MΩ, considerably greater
than those of analogue instruments. This makes the connection of a DMM as
voltmeter to a circuit less likely to distort existing current flows and potential
differences. They can be used for measuring potentials in electrochemistry.
Accuracy
Analogue meters for bench use in schools typically have an accuracy of 2% fullscale deflection. The accuracy of a DMM is usually greater. It is expressed less
simply, typically quoted as ‘basic x% accuracy ± y digits’. Consider an accuracy
stated as ‘2% ± 1 digit’ applied to a 50 V reading when set to a 200 V range: 2% of
50 V is 1 V and the least significant digits on this range are 1 V. Hence the true
value lies between 50 ± 1 ± 1 V, that is between 48 and 52 V. It is often necessary
to make a calculation like this to determine the accuracy of a particular reading.
Resolution
DMMs are often said to have a ‘higher resolution’ than analogue meters. The
resolution is the smallest change a meter can display and the DMM will display
smaller changes than can be seen with a pointer. This leads to several additional
and valuable uses: a DMM can sometimes be used in place of a light-beam
galvanometer or an electrometer. See section 12.3.4 (New uses).
Range
Because of this high resolution, a single range on a DMM will often allow a wide
range of values to be measured with sufficient accuracy, even with the 3 1/2 digit
displays on most inexpensive DMMs. (‘1/2’ means that the most significant digit
cannot be any of the full range of 0 - 9 but can only be blank or 1: ie, a three digit
display could read from 1 to 999, a 3 1/2 digit display from 1 to 1999.)
Electrical
robustness
DMMs are not damaged by the overloads likely in normal pupil work. When set to
voltage and resistance ranges, they will withstand voltages well in excess of the
maximum outputs of bench supplies while the current ranges, except for 10 A
ranges, are protected by fuses. DMMs will withstand reverse connection and
indicate it with a minus sign.
Visibility
The half-inch (13 mm) liquid crystal displays on most inexpensive DMMs are
visible, in suitable light, over two or three metres: DMMs can therefore be used for
some cases in place of special demonstration meters.
Relevance to the DMMs are used in industry, repair shops etc so that familiarity with them will be
useful.
outside world
Disadvantages
Reliability
The DMMs schools are likely to buy are at the low end of the price range and it
must be expected that a few will fail; like analogue meters, repair is unlikely to be
economical. It is recommended that meters are used as much as possible within
the guarantee period. The sockets and switches on some models may not stand up
to pupil use.
Too many figures With a display of up to four digits, pupils using DMMs will often quote more
figures than can be justified; judging the right number will need to be taught.
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© CLEAPSS 2005
Batteries
Most DMMs use PP3 cells; occasionally two AA (pentorch) cells are used With
alkaline cells, a life of up to 1000 hours use is claimed for some models but
200 hours is the more usual claim. A cell will not give a constant life irrespective
of the pattern of use; clearly, cells will deteriorate, albeit slowly, over the long
periods they are left unused. Alkaline cells have a longer shelf life and so it is
worth experimenting with them. It is important that DMMs are not put away
switched on. Rechargeable cells are not suitable for powering DMMs.
Confusion
DMMs can offer the user many options: in addition to more than twenty V, I and
R ranges, there are diode test facilities, ‘Low ohms’, ‘Memory’ etc.
Less teacher
control
By issuing pupils with a single-function meter or a multipurpose meter with a
shunt or multiplier attached, a teacher can ensure that the right range is being
used. This control is lost with a DMM which, typically, has over 20 ranges. Misuse
is unlikely to damage the DMM but, for example, using the 10 A range (unfused)
instead of the 20 V range could cause problems in the circuit being investigated,
for example, discharging a battery or overloading a device.
Rapid and
approximate
changes
A DMM is unsuitable for use as a galvanometer to show the induced emf in a coil
when a magnet is moved in and out of it or for showing the decay in the current
through a resistor across a charged capacitor; the meter cannot take readings fast
enough.
A pointer instrument can give a ‘current, no current’ comparison or rough orderof-magnitude indication or comparison better. For example, for some pupils, the
DMM would be a less appropriate instrument for showing that the current
through two bulbs in parallel is approximately double that through one.
12.3.2
Provision of digital multimeters
Class sets of DMMs are probably appropriate for general use from Year 9 upwards, for
some pupils at least. For some purposes, analogue meters will still be needed but
schools might decide to make DMMs their standard class meters and use their
remaining stock of analogue meters for these infrequent occasions.
DMMs are too complicated for junior forms and perhaps for older, less-able pupils;
however, if used, the switch(es) and unneeded sockets should be covered with a piece
of hardboard or card held on with rubber bands. For these pupils, schools may prefer
inexpensive analogue meters or single-range digital meters. Analogue and singlerange digital meters are considered in section 10.3 (Meters).
12.3.3
Which DMMs to buy
Guidance can be found in CLEAPSS Guide R105, Digital Multimeters. The models of
DMMs available on the market vary rapidly with time. DMMs often are alike but have
different trade names or look the same but have slightly different features and so
schools are advised to check very carefully what they are buying; in particular, they
should look closely at the ranges. The cheaper DMMs usually have adequate dc
voltage ranges but some may not have the 10 A current range essential for work
involving heating and magnetic effects.
If electronics is taught, it may be important to consider its needs when choosing
meters, ie, to look for suitable low current ranges.
Ranges and functions
DC volts
Almost all DMMs have the following ranges: 200 mV; 2, 20, 200 and 1000 V. One
or two lack the 200 mV range. The resolution on the 2 V range is 1 mV so that the
omission of the 200 mV range may not matter. However, it would be difficult to
measure the emf of a thermocouple without this range.
© CLEAPSS 1992
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DC current
What is available varies much more. Several have the following ranges: 200 µA; 2,
20, 200 mA; 2, 10 A; however, some have only three or even two current ranges.
Both 2 mA and 10 A ranges are necessary, with at least one intermediate range.
AC ranges
These vary from DMM to DMM; in some, ac ranges are the same as the dc ranges
with one or two omissions but some have no ac current ranges. Some DMMs with
several ac ranges are needed for work on transformers and it might be useful to be
able to meter ac used to power a heating coil: both 20 mA and 10 A ranges are
useful. However, most school work is with dc so that it may not matter if most of
the meters to be used by a class have limited ac ranges.
Resistance
ranges
The following ranges are standard: 200 Ω; 2, 20, 200 kΩ; 2 MΩ. Several have an
additional 20 MΩ range, some a 20 Ω range. The 20 MΩ range is valuable for
electronics but a 20 Ω range is more useful for general work.
Sockets
Most but not all meters have standard 4 mm sockets so that ordinary leads terminating in 4 mm plugs can be inserted.
It has been found that fuses in the meter are less likely to be blown if there are
separate sockets for current and voltage ranges. All meters with the 10 A range
necessary for schools will have separate sockets. The lower-current input will be
protected with a fuse.
Accessibility of
fuses
Pupils tend to blow the fuse protecting the lower current ranges. It is sensible to
buy DMMs in which the fuse is readily accessible.
Other functions
DMMs frequently have other functions, for example for testing continuity, diodes
and transistors, for measuring capacitance, frequency etc. With the possible
exceptions of ‘continuity testing’ (the meter bleeps if there is continuity) and ‘hold’
(the display remains unchanged until a button is pressed), these facilities are an
additional complication during general use and so are to be avoided if possible.
They may be of use during electronics work.
Recommended switching for class DMMs
The functions and ranges of DMMs can, depending on the model, be selected with one
or two rotary switches or push-button switches. With some, the selection of the range,
as opposed to the selection of the function (ie, voltmeter, ammeter or ohmmeter), is
achieved automatically; they are called autoranging.
Autoranging or
fixed ranges
Some believe that autoranging models are more suitable for pupils. Range
switching is not required, the most suitable range is selected automatically and
the full unit appears on the display, making it harder for pupils to make errors.
However, it is essential that other, more valuable features such as the size of the
display are not sacrificed for autoranging. Also, pupils may learn more about
magnitudes and units if they have to select the range. With autoranging, some
pupils may notice and record the unit initially displayed but not notice if the
range changes.
Rotary or pushbutton switches
DMMs controlled by a single rotary switch (with one or two buttons for special
functions) are clearer to understand and easier to use than push-button models
and likely to last longer. Push-button models can be used with one hand but this
is irrelevant for bench use.
In the sixth form, it is sensible if students have experience with more than one type of
switching.
12.3.4
New uses
These ideas are extracted from an article by Mark Ellse1.
1
Ellse, M, Electronic instrumentation in A-level physics, School Science Review, 67 (240), March 1986, pp 495-500.
Mainly Physics
As a nanoammeter:
measurement of
small currents
1210
© CLEAPSS 1992
With a 10 MΩ impedance, the 200 mV range of a DMM near full-range reading
takes a current approaching 20 nanoamperes, 20 nA. The resolution on this range
is 0.1 mV, corresponding to a current of 100 picoamperes, 100 pA. This sensitivity
is adequate for most school requirements, for example, detecting the ionisation
current produced by a 185 kBq (5 µCi) alpha source. Thus a DMM can be used
instead of a spot galvanometer or electrometer for some purposes.
At least one inexpensive meter has a 20 µA range with 10 kΩ. It may be suitable
for some current-measuring applications.
As a coulombmeter
A charge measurer or coulombmeter can be made by connecting a voltmeter across
a suitable capacitor of known value; however, the voltmeter must have a high
impedance or else the charge will leak away too quickly. With an impedance of
10 MΩ, a DMM across a capacitor of 10 µF has a time constant of 100 seconds; ie,
the pd will fall to 1/e (= 0.37) of its initial value after 100 s, giving adequate time
to take a reading (in 3 s, the reading will have dropped by not quite 3%). The
200 V range will give a full range reading for 2 mC, the 200 mV range for 2 µC.
Electrolytic capacitors, which have significant leakage and are polarised, are
unsuitable for this purpose; metallised film capacitors can be used and are
available from RS Components and other suppliers.
Adding up
potential
differences round
a circuit
It is useful to be able to add up the potential differences round a circuit and show
that their sum is zero, if due attention is paid to sign. A DMM enables the
potential differences across devices such as batteries and bulbs and even the wires
to be measured with sufficient resolution and with sufficient linearity for the
exercise to be convincing.
12.4
Dynamics
12.4.1
Introduction
The basic concepts of dynamics form an important part of school science. Initial
experimental work aims to establish the concepts of velocity, acceleration and mass.
These are linked in further work on Newton’s Laws of Motion, followed by that on
momentum and energy.
The main apparatus used is the dynamics trolley with its associated runway or track;
there should be enough sets of these for class work. Schools often have a linear air
track; some have ‘frictionless’ tables on which two-dimensional motion can be demonstrated. Outlines of how various ideas can be investigated with trolleys or a linear air
track are given in Table 12.2 below.
Additional equipment is needed for measuring g, the free-fall acceleration due to the
earth’s gravitational field. Finally, some schools will have apparatus for demonstrating angular motion.
12.4.2
Timing methods
There are three methods of timing moving trolleys, air-track vehicles and pucks:
tickertimers, stroboscopic photography and light gates. The development of
microcomputers has increased the popularity of the last method but the tickertimer is
probably the only device that could be available in sufficient numbers for pupil work.
Some teachers claim that pupils have difficulty in deriving values for acceleration
from tickertape; while this is true, it is probably due to a failure to understand
acceleration itself. A computer will give values for acceleration with less trouble but
will not reveal a failure to understand the concept.
© CLEAPSS 1992
Table 12.2
Concept, law
principle, etc
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Some basic investigations with trolleys or a linear air track
Apparatus1
Constant
velocity
Constant
acceleration
Trolley on fc2 track
Vehicle on la2 track
Acceleration
proportional to
force
Acceleration
inversely proportional to
mass
Elastic
collisions
Trolley on fc track
Vehicle on la track
Elastics etc to apply force
Trolley on fc track
Vehicle on la track
Extra trolleys, or masses
equal to masses of vehicles
2 trolleys on fc track
2 vehicles on la track
Extra trolleys, or masses
equal to masses of vehicles
Procedure
The vehicle is started and makes equally spaced dots on
tape, equal time intervals at light gates etc. Discussion of
friction compensation.
Repeated with track tilted. Experience in measuring acceleration.
Multiples of a force applied in turn with the vehicle pulled
along with 1, 2, 3 … elastic cords or springs stretched the
same amount.
A single, constant force is applied in turn with an elastic
cord to 1, 2, 3… masses. This can be achieved by stacking
trolleys on top of each other or adding equal masses.
One trolley with spring-loaded plunger extended is pushed
towards the other which is stationary. Each has a tape
through the timer (with two carbon discs if carbon disk type.)
Repeated with varied masses. Two repelling magnets can
be used instead of the plunger.
Two la vehicles are collided, fitted with elastic collision
fittings. Repeated with varied masses.
Totally
2 trolleys on fc track
One trolley fitted with a needle is pushed towards the other
inelastic
2 vehicles on la track
which is stationary and fitted with a cork. Each has a tape
collisions
Extra trolleys, masses equal through the timer (with two carbon discs if carbon disk type.)
to vehicles
Repeated with varied masses. Two attracting magnets can
be used instead of the needle and cork.
Two la vehicles are collided, fitted with inelastic collision
fittings. Repeated with varied masses.
Explosion
2 trolleys on uncompensated The spring-loaded plunger on one trolley is loaded. Trolleys
between two track
are put together and peg tapped to release plunger.
trolleys
No tickertimer
Repeated with varied masses.
Trolleys pulled 2 trolleys on uncompensated The trolleys are linked with elastic cords, pulled apart and
together
track
released. They collide and are held together by a corkNo tickertimer
needle arrangement. Repeated with varied masses.
Kinetic energy Trolley on fc track
The vehicle is pulled back with a projection (ie, a peg in a
proportional to Vehicle on la track
trolley) against a stretched rubber band, elastic cord etc, is
2
Rubber bands which will
released and the velocity measured. Repeated for 1, 2, 3 …
velocity
stretch across track
elastics, vehicle pulled back the same distance each time.
Loss in PE
Trolley on fc track
A cord attached to a vehicle passes over the pulley to a
proportional to Vehicle on la track
mass. The vehicle is released and the velocity measured
gain in KE
Cord, pulley clamped at the after the mass has fallen a measured distance. Repeated
end of track, masses
with different distances and masses.
1.
By ‘vehicle’ is meant either a trolley or a linear air track vehicle. For each experiment, it is assumed that there is
appropriate timing equipment: tickertape, light gates, stroboscopic photography etc.
2.
‘fc’ means ‘friction compensated’. The track is adjusted with window wedges etc so that a trolley travels at a constant
speed. ‘la’ is ‘linear air’ as in ‘linear air track’.
See also: Revised Nuffield Physics Teachers’ Guide Year 4 , Longman, 1978, ISBN 058204684X, p6 et seq.
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Tickertimers
A vibrator actuated by 2 - 12 V ac marks a paper tape drawn by a moving vehicle. The
distances between successive pairs of marks are the distances the vehicle travels in
0.02 (1/50) second (usually) and so are proportional to the velocity of the vehicle.
Lengths corresponding to constant time intervals, eg, 10 marks, are used to give
comparisons of velocity and hence accelerations.
Because the lengths of tickertape compared are the actual distances a trolley has
travelled, tickertape is a concrete way of showing motion. However, its appearance is
contrary to the intuition of some pupils: if two tapes representing different velocities
are compared, the lower velocity has the more closely spaced marks. Because this
looks ‘busy’, it is contrary to intuition which is perhaps why pupils find tickertape
confusing.
Further, tickertimers are noisy and not very reliable.
Successive
lengths of
tickertape
showing
acceleration
Marking of the
tape
Tickertimers can mark the tape by a point hitting a carbon paper disk above or
below it. The disk is free to rotate so that the point hits different positions on the
carbon paper which therefore lasts longer. Another model uses a ballpoint pen.
Recently, self-marking tape has become popular; this is impregnated with a
chemical which produces a mark on impact. This is much less trouble, avoiding
the carbon paper disk. However, self-marking tape is prone to tear and can pick
up writing from the opposite page if stuck in a notebook. Because it tends to fade,
it is sensible to mark the points with a pen.
Adhesive or non- Plain tickertape can be obtained gummed or ungummed. Since pupils will need to
adhesive tape? stick lengths of tape in their notebooks, gummed tape, which merely has to be
moistened, is much more convenient. Regrettably, gummed, self-marking tape is
not available.
Adjustment of
tickertimers
Most tickertimers work off 12 V ac but one model works off 2 V. Adjustment of the
spring can be critical in some models and small adjustments to the supply voltage
can sometimes improve the clarity of the marks.
Electronic methods of timing
Automatic, electronic methods have been in use for years, since Nuffield Physics
introduced the scaler-timer. However, the scope of these methods has expanded with
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the introduction of microcomputers, dataloggers and special timing units. They
depend on switching devices, eg, by which objects are released and switches opened or
closed simultaneously, and on light gates. A light gate is a switch consisting of a lamp
and a light-sensitive device (diode or phototransistor). A card (called an ‘interrupt
card’) on a moving vehicle passes between the two parts of the gate and the interruption of the light beam is automatically timed by one of several means: a milli- or
centisecond timer, a special velocity-acceleration unit, a datalogger or a microcomputer.
Some switches can be bought from suppliers: eg, for measuring g by the free-fall of
Switches for
automatic timing a metal ball. Some can be constructed in minutes, using contacts of aluminium
foil; others take longer to make.
Aluminium
foil
Circuit
completed
by ball
Tape
Time of impact of a toe on a ball
Magnet
Aluminium
foil
Contact
Hinged trap
Commercial g by
free-fall apparatus
Light gates
Clothes-peg
release switch
There are several models of light gate on the market; not all will be compatible
with all timing devices. For trolleys and air tracks, it is very convenient to have
the lamp and photosensitive switch permanently mounted on the same rod so that
they do not have to be realigned each time they are used. However, if the timing of
movements of other objects is expected, then separate units which can be placed
some distance apart might be more useful.
If commercial gates are unsuitable or too expensive, it is possible to construct
light gates. For example, a reflective opto-switch (eg, from RS Components) can be
used with a reflecting tape attached to the object to be timed.
Interrupt cards
These are cards which are attached to moving vehicles such as dynamics trolleys
or air track vehicles to interrupt light gate beams; they are best made of thin
black card. From the length of a card (typically 50-150 mm) and the time of interruption, the velocity of the vehicle can be calculated. Because the light beam has
finite width, the length of the card will not give the true velocity: the effective
length (see diagram below) can be found by experiment. However, the precise
velocity is needed only in attempts to measure g, the acceleration due to gravity.
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A double-interrupt card can give readings of uniform acceleration but to do so
needs a timing device which can record two readings in rapid succession.
Motion
d
s
d
d
Interrupt card
Double interrupt card
s
Section through lightbeam
(diameter exaggerated)
2
Acceleration =
Effective length of card is somewhere
between s and s + 2d
v - u
2s
( dt ) - ( dt )
2
2
=
1
2
2
2s
Where t 1 is the time for the first
interruption, t 2 for the second
Modes of timing
Basically there are two modes of timing: duration, in which what is timed is the
interval during which a switch is closed (or opened) or a light beam interrupted, and
triggered, in which timing is started by a switch opening (or closing) and stopped by
the switch opening (or closing) again. (It can, of course, be triggered by the start of a
light beam being interrupted etc.)
Duration control
Timing takes place only while an input is completed or broken. For example, the
time the toe of a shoe is in contact with a football is measured by completing a
circuit attached to the timer input with contacts on the shoe and ball; the
instrument times while these two contacts touch. A reaction time can be measured
by an experimenter closing a switch (which also lights a lamp) to the input; the
instrument times until the subject opens a switch in series. The velocity of a
linear air track vehicle can be found by recording the time taken for an interrupt
card, whose length is measured (eg, 50-150 mm), attached to the vehicle to break
a light beam falling on a photocell attached to the input.
Triggered control Some automatic timing devices have triggered control: timing is started or
(pulsed or gated stopped by the momentary completion or breaking of a circuit, typically by the
momentary breaking of a light beam. To measure the time taken by a linear air
control)
track vehicle to travel between two points 250-1000 mm apart, light gates are
arranged across the track at the two points. Timing starts when the first beam is
interrupted but subsequent restoration of this beam will not stop it; only
interrupting the second beam will.
Instruments fitted with triggered control can usually also be used for duration
control. Selection of the type of control is made with a switch or choice of sockets.
Triggered control is useful as it allows a greater range of dynamics experiments
(see Table 12.3) but it is not essential; there is nearly always another way of
illustrating the same relationship. It is not difficult to build a unit to convert
duration control to triggered control (circuits are available from CLEAPSS).
Millisecond timers, scaler-timers
Many schools will have millisecond timers, in some cases combined with scalers (pulse
counters) as scaler-timers; see section 12.10.5 (Measuring apparatus). All will be fitted
with input sockets which allow duration (make and break) control; some will also
allow triggered control.
© CLEAPSS 1992
Table 12.3
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Examples of the use of different methods of timing
Measurement
Control of d/c/m*
timer
Note
Timer used as a stopclock
manual
d
Often a switch. If not, connect a switch across the
‘make’ sockets.
Human reaction times
duration
c
Various possibilities: eg, the experimenter’s switch
across ‘make’, the subject’s across ‘break’.
Acceleration due to gravity, g
by free fall
duration
c
Releasing the ball between contacts opens ‘break’.
The ball falls and hits a trap which opens ‘make’.
s=1/2 gt 2
Time of impact of a toe on a
ball
duration
m
Aluminium foil contacts on toe and ball are connected
across ‘make’.
Velocity of an air rifle pellet
duration
m
Pellet breaks two foil strips, the first across ‘break’,
the second across ‘make’.
Timing a longitudinal wave
along a line of trolleys
duration
c
Contacts on end trolley and wall are connected
across ‘make’.
Acceleration of a dynamics
trolley
duration
light gate
c
Card on trolley. Two light gates connected in parallel
across ‘make’ some distance apart. First reading
memorised by the experimenter.
Velocity of a long pendulum
duration
light gate
c
Card attached to the pendulum. Light gate across
‘make’.
Velocity of air track vehicle
(catapulted with different
numbers of elastics)
duration
light gate
c
Card on vehicle. Light gate across ‘make’.
Time for vehicle to travel to
different points on a tilted air
track: to illustrate s = kt2
triggered
light gate
c
Pair of light gates, one across ‘make’, the other
across ‘break’ are placed successively at adjacent
points whose distances from the starting points are
proportional to n2, n = 1, 2, 3 …
Acceleration due to gravity, g, triggered
by free fall (or the acceleration light gate
of a trolley)
m
Pair of light gates, one across ‘make’, the other
across ‘break’, are placed one below the other as far
apart as possible.
s1 = ut1 + 1/2gt12
s2 = ut2 + 1/2gt22
Angular velocity of a turntable
c
Beams in a pair of light gates, one across ‘make’, the
other across ‘break’, are broken by a card on the rim
of a turntable.
*
triggered
light gate
d/c/m indicate the most appropriate minimum timing interval: d = 0.1 second; c = 0.01 second; m = 0.001 second.
Timing with computers, VELA and single-purpose units
Interfaces for timing are available; these use the microcomputers commonly found in
schools and are supplied complete with software and documentation. Some time with
an uncertainty of 30 microseconds, making possible accurate determinations of g by
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© CLEAPSS 2005
free fall. They can record readings taken in rapid succession and so allow the use of
double-interrupt cards.
VELA can also be used for timing. Also available are single-purpose time-velocityacceleration units to which switches can be connected and with displays giving values
of times, velocities and accelerations. These are very convenient to use but, unlike a
computer, cannot be used for anything else.
All these devices are provided with full instructions.
Motion sensor
This is an ultrasonic rangefinder which is used linked to a microcomputer, enabling
graphs of distance, velocity and acceleration against time to be displayed as the
motion takes place. It can be used to display the motion of trolleys and linear air track
vehicles but its unique value lies in its ability to display the motion of pupils, helping
them to develop a ‘feel’ for various motions. It is available from Educational
Electronics with the necessary software and detailed instructions.
How it works
An electrostatic transducer emits a train of 16 pulses at about 50 kHz. This is
reflected by the object whose motion is being observed and recorded by the same
transducer. Wave trains are emitted at 20 Hz, with the receiver inhibited for a
period after each train so that only reflections are recorded. The operational range
is about 35 cm to 6 m. Targets with a width of 1 cm or more can be observed. The
ultrasonic beam is narrow, reducing interference from false reflections1.
Stroboscopic photography
With a xenon stroboscope, it is possible to take stroboscopic pictures of moving objects
if a Polaroid camera is still available. Stroboscopic photography is essential if
measurements are to be made on two-dimensional motion on an air table.
Use with air
tracks
12.4.3
For example, momentum changes caused by collisions of various kinds between
vehicles can be shown. Drinking straws attached to the two vehicles show up as a
series of lines on the photograph and measurements can be made to confirm the
Law of Conservation of Momentum. In the past, air tracks were provided with
screens which could be moved quickly after a collision to block the camera’s view
of one vehicle and simplify the photograph. They are no longer supplied, probably
because it is much simpler to make measurements using a light gate and timing
device.
The air track
Similar models are available, complete with accessories, from the normal suppliers.
An air track should be about 2 m long, which may present storage problems, particularly as great care must be taken to prevent a track being bent. While blowers can
sometimes be devised out of cylinder vacuum cleaners, it is convenient if the blower is
as quiet as possible; there should be a bleed valve, which leaks air, allowing control of
the air supply to reduce friction on the vehicles to the minimum. Full instructions are
provided.
12.4.4
Frictionless tables
Frictionless tables are used to show two-dimensional motion. There are two kinds: the
first is the air table, a hollow chest with a perforated and very flat top connected to a
1
This note is based on SSERC, Evaluation Report - Motion Sensor, Science & Technology Bulletin, 163, 1989, p37.
© CLEAPSS 2005
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blower, the second a sheet of flat glass with a suitable rim. The former is more
expensive but does not require solid carbon dioxide or pucks fitted with air pumps. It
can be operated with the blower used for a linear air track. Tables are usually 750800 mm square with the top surface black to aid photography.
Types of puck
For air tables. Only simple, flat disk pucks are required; these can have magnetic
rims, useful for producing elastic collisions, or brass rims for partially inelastic
collisions.
For glass plates, pucks generating their own cushions of gas are required. Pucks
can use solid carbon dioxide and consist of a circular rim with a top: solid carbon
dioxide is placed under the puck where it generates a cushion of gas. Their
disadvantage is the need to obtain a supply of solid carbon dioxide; section 11.2
(Cryogenics) gives some help. Pucks with battery-operated pumps are available
which generate their own cushions of air. While these work well, they are not as
reliable in the long term as the other kinds.
12.4.5
Trolleys and runways
Trolleys
Similar trolleys are available from the main suppliers. They are robust but care must
be taken with them if they are to continue to work well. Full details of how trolleys
can be used are given in Nuffield Physics texts1. They are provided with a number of
features and require certain accessories.
Stacking
Trolleys are made to stack so masses can be changed in simple multiples: ie, one
trolley, two trolleys etc. Short rods fitted into holes in one trolley allow another to
be fitted on top. Metal plates with the same mass as a trolley are available as a
cheaper substitute.
Elastic cords with These are the recommended method of applying forces in simple multiples. An
eyelet at one end of a cord is dropped over a rod at the back of a trolley. The other
eyelets
eyelet is held by finger and thumb and kept over a mark near the front of the
trolley, thereby stretching the cord an approximately constant amount so applying
an approximately constant force. To apply twice the force, two cords are used and
so on. While this method of applying force requires a knack, it does not have the
logical difficulty that using a thread passing over a pulley to a mass does: ie, the
mass being accelerated is not just that of the trolley but includes the falling mass.
Cords can be bought ready-made or constructed from shirring elastic from a
haberdashery department. However, it is difficult to get them all the same length.
It is important to prevent them tangling, perhaps by storing each group’s set
threaded on two nails on a strip of wood. Because elastic cords have a limited life,
some prefer to use springs.
Accessories for
momentum
changes
Trolleys are fitted with rods and springs inside holes running down their lengths;
these can be fired by pressing a rod through a hole, thereby pushing two stacks of
trolleys apart. It can be shown that the net momentum change is always zero.
Trolleys can be made to collide in different ways: elastically by attaching magnets
to the fronts of a pair of trolleys so that they repel; totally inelastically (stuck
together) by attaching a cork on one and a spike on the other or with two magnets
arranged to attract.
Wave
demonstrations
1
A series of trolleys can be linked with springs to make wave models. Special
attachments are needed for this. For more details see section 12.21 (Wave
machines).
Revised Nuffield Physics Teachers’ Guide Year 3, Longman, 1977, ISBN 058246831, p24.
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Runways
These are usually made from blockboard mounted between two lengths of slotted
angle-iron whose purpose is both to protect the blockboard and keep it straight and
also to prevent trolleys rolling off. Lengths vary from 2.5 - 1.5 m; the longer runways
give better results but are harder to store. It is sensible to tie a length of string across
the ends of runways to prevent trolleys falling off.
Friction
compensation
12.4.6
Runways are compensated for the friction of the trolley by tilting; a few exercise
books under one end is usually adequate.
Other dynamics equipment
Air rifle experiment
An air rifle or air pistol is sometimes used in dynamics experiments. The velocity of
the pellet can be found by it successively breaking two aluminium strips some
distance apart and connected to a timing device. A pellet can be fired into a lump of
Plasticine on an air track vehicle and its velocity calculated from the subsequent
velocity of the vehicle, using the Law of Conservation of Momentum. The two values
for velocity can be compared.
rifle can be obtained from the normal suppliers mounted on a stand. It should be
m The
kept mounted on its stand and always pointed away from people, particular attention
being paid during loading. A safety screen should be placed between the class and
targets, gates etc. An open cardboard box, on its side, lined with some soft material
should be placed beyond the target.
All present should wear eye protection.
Free-fall
A device, called a ‘falling bodies’ or ‘projectile’ apparatus, is available from the normal
suppliers and used to teach the independence of vertical and horizontal motion. If a
catch is pressed, two steel balls are released simultaneously: one just drops, the other
is given a horizontal velocity. They can be heard to hit the ground together.
The acceleration due to gravity, g, can be measured using a timing device and a pair of
switches obtainable from the normal suppliers; see the section 12.4.2 (Timing
methods) above.
Timing ball
This is a hard ball with a stitched cover, resembling that used in baseball, which is
similar in size to a cricket ball. Inside is a digital clock plus a battery which lasts
about five years and cannot be replaced. If a marked area is pressed, the clock is
zeroed; when it is released, the clock counts in milliseconds until the ball hits the floor
etc. The elapsed time can be read through a small window in the ball. The ball should
not be dropped from a height of greater than 2 m or thrown against a hard surface. It
can be used for demonstrating g by free-fall and is obtainable from Unilab Ltd.
Rotary motion
For experimental work on rotational dynamics, tables which rotate with low friction,
some using air bearings, can be obtained. It is possible to make a d-i-y table using a
bicycle wheel bearing which can be used for some aspects. The relationship between
centripetal force, mass and velocity (F = mv2/r) can be tested with the simple
apparatus shown in the diagram below. The tube is held in the hand and the bung
whirled round horizontally, the angular velocity adjusted until the paper clip on the
cord is just below the tube. The edges of the tube should be rounded; if a glass tube is
© CLEAPSS 1992
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used instead of plastic, it must be protected by enclosure in rubber tubing or covered
with tape. For further ideas, see the Nuffield Physics Guide to Experiments, Year V.1
Bung
Plastic tube
Paper clip
1 - 2 g masses
or washers
12.4.7
Other means of recording linear motion
SSERC has developed methods using two devices with voltage outputs directly
proportional to distance in the first case and velocity in the second. The first depends
on a multi-turn potentiometer, the second a small electric motor, both inexpensive.
Further details are given in CLEAPSS guide L97, Distance, Time & Velocity2.
12.5
Electrolytic capacitors
capacitors can explode if misused. It is important to make sure that any
m Electrolytic
electrolytic capacitor:
(a)
(b)
(c)
(d)
is not connected the wrong way round, ie, with the wrong polarity;
does not have a voltage applied to it above its rated value;
does not need re-forming;
is not expected to carry a ripple current above its rated value (in a smoothing
circuit).
Re-forming
Electrolytic capacitors depend on a very thin insulating layer formed electrolytically between chemical layers inside the capacitor. After 12-18 months without use,
this layer can need re-forming which is done by connecting the capacitor to a
smooth dc supply (positive to positive). The voltage is then increased slowly from
zero to about the rated voltage, taking about one minute. The capacitor is then
disconnected and has been re-formed.
1
Nuffield Physics Guide to experiments V., Longman, 1968.
2
The material in this guide originally appeared in the SSERC Bulletin; it is reproduced by kind permission of the Director.
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Choice of
electrolytic
capacitors for
smoothing
circuits
12.6
1220
© CLEAPSS 2005
The rated voltage of a capacitor must be greater than the maximum dc voltage
applied to it. Reliability is improved if it is considerably greater; it is sensible to
aim for twice the maximum.
The ripple current rating must also be adequate. A smoothing capacitor can be
thought of as short-circuiting an ac ripple imposed on the dc load current. Estimated values of the ripple current are 1.4 × load current for full-wave rectification,
2.8 × load current for half-wave rectification. Again, for reliability, it is sensible to
choose a rated value well above the estimated value.
Electron-beam and similar tubes
These are used for teaching thermionic emission, properties of electrons and electron
beams, gas discharge and dynamics. Operating with a maximum voltage of 5000 V,
they do not offer any radiation hazard. See also guidance leaflet PS 76 Electron-beam
tubes: an introduction. While some schools may have Leybold tubes imported some
years ago, most will have Teltron tubes as described in Table 12.4.
Table 12.4
Voltage &
current
requirements of
Teltron tubes
Type of tube
Gas discharge
Voltage needed
Approx current
Supply needed
2000 - 5000 V
1 - 2 mA
Planar diode
500 V
3 mA
EHT + filament supply
Planar triode
500 V
0.35 mA
EHT + filament supply
Luminescent
2000 - 5000 V
2 mA
EHT + filament supply
Maltese cross
2000 - 5000 V
2 mA
EHT + filament supply
Perrin
2000 - 5000 V
2 mA
EHT + filament supply
Deflection e/m
1500 - 5000 V
1 mA
EHT + filament supply
Electron diffraction
1500 - 5000 V
0.2 - 0.4 mA
EHT + filament supply
Double-beam gasfilled
300 V
10 - 20 mA
HT (includes filament supply)
Gas-filled planar triode
300 V
10 mA
HT (includes filament supply)
50 V
2 mA
Variable LT supply + filament
supply
Critical potentials
EHT
It will be noted that an EHT supply can be used for almost all the tubes. EHT
supplies available for schools have their currents limited to 5 mA and so are safe.
m
An HT supply giving 300 V at more than 5 mA is required for two tubes, including
the double-beam tube which is popular. School HT supplies can often provide currents up to 150 mA and are dangerous. See section 12.9 (High voltages).
Filament supply
This will be needed where indicated and is normally 6.3 V ac with a maximum of
7.5 V. However, the electron diffraction tube benefits from a variable supply: 5 9 V ac or dc and the critical potentials tube requires 5 - 7 V ac or dc.
Other power
supplies
Additional power supplies will be needed for certain tubes: the planar triodes, e/m,
double-beam and critical potential tubes. See the supplier’s literature.
All connections are made with 4 mm plugs or free 4 mm sockets, ie female conPlugs for HT &
EHT connections nectors on the end of leads. For high voltages, plugs should have shrouds, the sort
with a spring-loaded insulated cover which retracts as the plug is inserted. These
are essential1 with a supply providing a current of more than 5 mA at over 50 V.
Repairs
1
If a tube is damaged, it is always worth contacting UK 3B Scientific, the company
that now manufactures Teltron tubes, to see if repair is possible.
See Electrical Safety in Schools (Electricity at Work Regulations 1989) GS23, HSE, 1990.
© CLEAPSS 2005
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Disposal
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Sealed, evacuated vessels should be broken before disposal through the public
refuse system. Wear eye protection and gloves and break the vessel inside a stout
cardboard box. Pack the remains in an old cardboard box before disposing in the
refuse. Consult CLEAPSS if a tube contains an active metal such as sodium.
If a Teltron electron-beam tube needs to be disposed of, UK 3B Scientific may
accept returned tubes.
12.7
Electronic meters
This section is concerned with dc amplifying meters, ie:
coulombmeters;
sensitive galvanometers (micro and millivolts, nano and microamps);
picoammeters;
high-impedance voltmeters with 1 V or 2 V ranges; and
combined meters covering all the functions offered by the four instruments listed
above.
The use of digital multimeters for measuring these functions will also be referred to.
12.7.1
The different meters described
The coulombmeter (Nuffield1 specification 1512) measures charge and possibly
replaces an electrometer/dc amplifier with a capacitor across it. However, it can also
be used for some purposes instead of a gold-leaf electroscope. Coulombmeters are
available as single-function meters and as one function of a combined meter.
The sensitive galvanometer (1101) is a replacement for the taut-band spot galvanometer. This has become expensive to repair and its electronic replacement is direct
reading and more robust. Some models can integrate current over time, so enabling
the meter to be used with a search coil. Sensitive galvanometer functions are provided
by separate meters such as microvoltmeters or by combined meters.
The picoammeter (1516) replaces the electrometer/dc amplifier fitted with a shunt.
Types are available as single-function meters and as one function of a combined
meter.
The high-impedance voltmeter (1509) is a replacement for the electrometer/dc
amplifier used to measure potential differences of 1 or 2 V. High-impedance voltmeter
functions of this range are available only as some of the functions of combined meters.
Coulombmeters
A coulombmeter is a very high-impedance voltmeter, often digital, connected across a
capacitor with a very high leakage resistance. It is calibrated in nanocoulombs. A d-i-y
version can be made with a capacitor across a digital multimeter.
The Nuffield specification stipulates a resolution of at least 1 nanocoulomb (nC) and a
range of at least 100 nC. Those available have a range of 0 - ±200 nC with a resolution
(smallest reading) of 0.1 nC or 0 - ±2000 nC with a resolution of 1 nC. Capacitances
vary from 0.1 to 5 µF.
These coulombmeters were designed for three Nuffield experiments: see section 12.7.2
(Experiments using the meters). Coulombmeters can also be used for elementary
electrostatics demonstrations and, for some, are better than a gold leaf electroscope.
1
Most of these instruments were introduced by the Nuffield Revised Advanced Physics course and are made to their
specifications; see the Apparatus Guide and other guides, published by Longman. See also Ellse, M, Electronic instrumentation
in A-level physics, School Science Review, 67 (240), March 1986, pp 495-500.
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A coulombmeter instead of a gold-leaf electroscope for electrostatics
A coulombmeter measures small charges while a gold-leaf electroscope is a very
Coulombmeter
and electroscope high-impedance voltmeter which can be calibrated to read kilovolts. Table 12.5
below compares the characteristics of a coulombmeter and an electroscope.
compared
Feature
Table 12.5
Capacitance
Features of
coulombmeters
& electroscopes Voltage range
Leakage resistance
Coulombmeter
Electroscope
High
(0.1 - 5 µF)
Low
(<1 pF)
Low
(<0.5 V)
High
(1 - 5 kV)
High
1012 Ω
Even higher
>1015 Ω
An outline of some experiments is given in section 12.7.2 (Experiments using the
meters) but some general points may be useful here.
Ease of use
Despite its very different characteristics, the coulombmeter is not significantly
easier to use for electrostatic experiments. The charges on polythene rods etc
remain the same as do the comparable stray charges on the experimenter’s body,
the induced charges on hands etc which can cause problems.
‘Black box’
The coulombmeter is a black box and it is not expected that junior pupils should
understand how it works; sixth formers need to understand only that it is a highimpedance voltmeter with a low leakage capacitor across it. Its use to demonstrate
that a charge is positive or negative is direct and simple. An electroscope is less
simple or direct but it can, however, be used to teach a number of ideas about
potential and charge distribution. A pupil who really understands how one works
understands electrostatics.
Technique
With good technique and luck, a demonstration with a coulombmeter can be very
convincing. Eg, if the tips of neutral acetate and polythene rods are rubbed and
inserted singly in a small can connected to a coulombmeter, equal but opposite
readings can be obtained. If the rods are then inserted together, the reading can
be zero. However, a small difference which would not show on an electroscope can
spoil the effect.
Uses of an electroscope where it cannot be replaced by a coulombmeter
Ionisation from a Because the potential difference needed for a full-range reading is less than a volt,
a charged coulombmeter will not discharge rapidly when held near a flame.
flame
Flame probe
The leakage resistance of a coulombmeter is too low to work with this device for
measuring potential and its voltage range is too low.
Polarity of the
charges
A coulombmeter cannot be used to demonstrate polarity of charges on a Perrin
tube, a Maltese Cross etc as potentials are too high.
Use of a digital multimeter as a coulombmeter
See section 12.3.4 (Digital multimeters, New uses.).
Sensitive galvanometers
Use for potential difference and current measurement
The Nuffield specification for a sensitive galvanometer (1101) stipulates the need to
measure potential differences between 1 µV and 1000 µV with an internal resistance
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of at least 20 Ω. The resistance should be as high as possible as it is for small potential
differences rather than small currents that the meter is most often required. In these
uses, the instrument can be considered as a microvoltmeter.
Replacement for
a spot galvanometer used as a
microvoltmeter
Meters now available usually cover a wider range than this and their resistances
are considerably above 20 Ω; they range from 1 kΩ to 1 MΩ. These values are high
compared to that of taut-band galvanometers such as the Edspot light-spot
galvanometer which these meters replace; also, these meters are more robust than
the Edspot and are direct reading. They employ a chopper-type microvolt amplifier to reduce the effects of drift and noise which would obscure readings if straight
dc amplification were used: the input signal is chopped into a series of pulses
which are amplified and then averaged.
The Edspot is widely used outside Nuffield courses but, with one exception, it is
difficult to think of any common uses for which an electronic microvoltmeter
would not be as good if not superior. (Uses are given in 12.7.2.) Users will have to
decide whether they are indeed trying to detect or measure small voltages but, if
it turns out to be a small current requiring a lower resistance meter, most of the
instruments considered have current (low resistance) ranges, ie with which they
can be considered as nanoammeters or microammeters.
Unless it is provided with a special integrating facility, an electronic galvanometer cannot replace the ballistic use of a spot galvanometer.
Replacement for
a spot galvanometer used
ballistically
Spot galvanometers have, on occasion, been used ballistically in schools for the
measurement of charge, both the static charge in a capacitor and the charge which
flows due to a change in magnetic flux linking the circuit (the current-time
integral).
This charge measurement can now be achieved more satisfactorily with other
instruments: eg, the charge in a capacitor can be measured with a coulombmeter
and a magnetic field with a Hall probe or using ac to produce a varying field and
connecting the search coil to an oscilloscope. However, at least one of the
instruments currently available measures charge by integration and so can be
used as a direct substitute for ballistic use of a spot galvanometer.
Use of a digital multimeter as a sensitive galvanometer
See section 12.3.4 (Digital multimeters, New uses.).
Picoammeters
The Nuffield specification (1516) stipulates a resolution of 10-11 A or 10-12 A. It is used
for measuring the current through an ionisation chamber and the current produced by
the photoelectric effect (see Experiments using the meters, below).
Some meters advertised as nanoammeters may be suitable and a digital multimeter
can be used for one of the ionisation chamber experiments.
High impedance voltmeters
The Nuffield specification (1509) asks for a full-scale deflection of 1 or 2 V and a high
input impedance. For two of the experiments (see Experiments using the meters,
below), a digital multimeter with the usual impedance of 10 MΩ will suffice; for only
one is a resistance of 100 MΩ an advantage. Schools may not wish to buy an
instrument just for one experiment but two combined instruments currently available
have this high-impedance voltmeter facility.
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12.7.2
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Experiments using the meters
Experiments using a coulombmeter
Electrostatics (Nuffield)
Charge is successively transferred on a metal plate with an insulating handle
B16: Spooning
from a conductor in the protected positive socket of an EHT supply to an electrode
charge
in the coulombmeter. The supply should be set at 1 - 2 kV.
EHT
EHT
nC
nC
B16
E6
E6: The charge To investigate the factors affecting the charge, the unearthed plate is first charged
on parallel plates by momentarily touching it with the flying lead . Then its charge is measured by
touching it with a stiff wire in the coulombmeter socket. Hands and other objects
in which charges could be induced must be kept away from the plates. Plate
separation and voltage are varied.
E8b: The value
of k in V=kQ/r
(r is the radius of a spherical conductor.) To measure k, the procedure is the same
as for E6.
Elementary electrostatics experiments
NB. A hair dryer or fan heater is useful for drying insulating rods, especially in damp
weather. The negative terminal of the coulombmeter should be earthed. An electrode
of reasonable size (eg, comparable to an electroscope plate) should be fitted to the
positive terminal. It is convenient for some experiments if the electrode will allow a
small tin can to be stood on it.
Presence of
charge
The presence of charge on a rubbed rod and the induced charge on different parts
of a conductor produced by induction is easy to show by bringing it up to the
electrode. Earthing the plate before removal leads to permanent charging by
induction.
Charging by
contact
Charge can be transferred by stroking a charged, insulated rod on the electrode.
The transfer is easier if a razor blade is taped to the electrode.
Identity of
polarities
It is possible to show that electrostatic and electrochemical polarities are
identical. Because coulombmeters operate at low potential differences, it is very
easy to show that, say, the carbon rod of a dry cell has the same effect on a
coulombmeter as a rubbed piece of acetate: therefore, both have the same polarity.
Charges are not
created
To show that positive and negative charges are separated rather than created,
polythene and acetate rods are first neutralised by passing over a small Bunsen
flame. The tips are shown to be neutral by insertion in a small tin on the
coulombmeter electrode. They are then rubbed together and reinserted, singly and
then together.
Distribution of
charge on a
conductor
It is easy to show that the charge is concentrated on the sharper edges and that
there is none inside. A small electrode on a insulated handle (a proof plane) is
touched on the conductor (a tin) connected to an EHT supply at 6 kV and then on
the coulombmeter; this is repeated for various points.
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EHT
nC
nC
Charge distribution
Faraday’s icepail
Faraday’s icepail A conductor on a long insulating handle is used (an expanded polystyrene sphere
stuck to a ball point pen case and then painted with Aquadag). The conductor is
experiment
charged to 6 kV and then carefully lowered into a tin on the coulombmeter. The
reading increases as it enters but does not change as it is touched on the bottom,
showing that the net charge inside a closed conductor is zero.
Experiments with a sensitive galvanometer
Potential difference measurements - as a microvoltmeter (Nuffield)
Electrical energy stored in a capacitor is converted to heat by discharge through a
B19: Energy
2
proportional to V heating coil. The microvoltmeter is used to measure the emf of a thermocouple
and hence give the rise in temperature of the bundle of insulated resistance wire
through which a 10 000 µF capacitor charged to 25 V is discharged. (Larger
capacitors are now available but with lower maximum voltages: a 1 F capacitor
charged to 6 V, however, contains more energy than the example above.)
D8: Superposition 1 GHz waves are superimposed in various ways and the interference patterns are
investigated with a simple diode detector in series with the microvoltmeter. The
of waves
wavelength of the radio waves is determined from the positions of the maxima or
minima.
H5a & H5b:
The Hall effect
The microvoltmeter measures the potential difference across a semiconductor slice
at right angles to both a current and to a magnetic field.
In both investigations, an approximation to a steady emf is maintained for a
H8 & H12: The
emf induced in a second or two as a conductor is moved between the poles of a magnet etc. It is
measured with the microvoltmeter.
moving wire
J18a & b; J21b: The microvoltmeter forms part of a simple 1 GHz receiver used to investigate
Experiments with guided waves, free waves and polarisation.
GHz waves
The microvoltmeter is used to measure the emf of a thermocouple and hence give
K10: Entropy
changes in a gas the change in temperature of air expanded in a bicycle pump with a reversed
washer.
Current measurement - as a nanoammeter (Nuffield)
The galvanometer is used to show that the shuttling ball does, in fact, carry
E1: A charged
charge which it averages out to give a current reading. Clearly, the resistance of
ball between
the meter is not important and the galvanometer can be thought of as a nanocharged plates
ammeter rather than a microvoltmeter.
E7: Factors
affecting the
charge on parallel
plates
A capacitor is charged and discharged continually using a reed switch. Again, it is
measurement of current rather than potential difference which is needed. See E1.
Depending on the size of the capacitor and the frequency of the reed switch, it is
possible for the resistance of the nanoammeter to be too high to allow complete
discharge.
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Other uses as a nanoammeter
Two well-insulated electrodes (4 mm plugs) are held in stands so their tips are a
Ionisation
few millimetres apart. They are connected in series with a meter set at 100 or
produced by a
200 nA, a protective resistance of a few tens of MΩ and an EHT supply. Care must
flame
be taken that earthed terminals on the supply and the meter are directly
connected. A match flame beneath the electrodes produces a reading. A digital
multimeter set as a voltmeter can also be used.
Conductivity of
polythene
This exercise is in the original Nuffield Advanced Physics course (Experiment
2.4). The current through a sheet of thin polythene is measured by applying 10 V
to films of Aquadag painted on opposite sides. A protective resistance is needed.
Ballistic experiments with the sensitive galvanometer
The charge in a Measured by discharging it through a spot galvanometer. Better done with a
coulombmeter.
capacitor
Magnetic field
strength
Measured by moving a search coil connected to the galvanometer out of the field.
Can be done only if the meter has a facility for integrating current over time.
Mutual
inductance
By measurement of the charge induced in the secondary by a known current
change in the primary. Can be done only if the meter has a facility for integrating
current over time.
Experiments using a picoammeter (Nuffield)
F2: Measurement From the ionisation current, the number of ion pairs produced by an alpha
of the ionisation particle can be found and hence an estimate of the energy of the particle. The
current produced ionisation chamber containing the source, an EHT supply and the currentmeasuring instrument are connected in series. If a 4 kBq (0.1 µCi) source is used,
by an alpha
then a meter with a resolution of 1 pA (10-12 A) is needed; with 185 kBq (5 µCi), a
particle
resolution of 10 pA (10-11 A) is adequate; a digital multimeter can be used.
EHT
pA
F7: The decay of The ionisation chamber, a source of 12 V and the current-measuring instrument
are connected in series. Radon is puffed into the chamber. A resolution of 1 pA (10radon and the
12 A) is needed for some radon generators. With a balloon on the outlet of the
determination of
chamber, it may be possible to inject more radon and use a meter of lower
its half-life
sensitivity.
L2: Simple
photoelectric
effect
About 6 volts is applied between a cleaned strip of magnesium ribbon and a
copper gauze bent in a cylinder round it. While the exercise is only a qualitative
demonstration, the current-measuring instrument should have a resolution of
1 pA (10-12 A).
Experiments using a high-impedance voltmeter (Nuffield)
B10: Highresistance
voltmeter
Two 220 kΩ resistors are connected in series with a 1.5 V cell. Voltmeters with
different impedances (moving-coil, oscilloscope, high-resistance voltmeter) are
connected in turn and together across one of the resistors, the point of the exercise
being to show that the resistance of a voltmeter can affect its reading. A digital
multimeter is quite suitable for this.
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K12: Concentration cell
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The emf generated by this cell with two different concentrations of copper sulfate
solution requires a range of 100 mV; 10 MΩ is sufficient resistance for the meter
and so a digital multimeter is satisfactory.
L3: Colour of light A high-impedance voltmeter is used to measure the potential difference across a
potassium photoemissive cell in a very simple circuit. The Nuffield Apparatus
and energy of
Guide suggests that, with some cells, the high-impedance voltmeter will not have
photoelectrons
a sufficiently high impedance and the electrometer/dc amplifier is necessary.
Other work with a high-impedance voltmeter
One can be useful for measurements on high-impedance devices such as
operational amplifiers.
12.8
Heat experiments
12.8.1
Calorimetry
See also section 9.4 (Calorimetry).
Heating coils
low-voltage immersion heaters used in calorimetry have been known to
m Certain
explode because steam has been generated inside them from water entering through
cracks. See section 9.10.8 (Electrical heating equipment).
Heat and power measurements
The most serious experimental errors in measurements involving heat and power are
those caused by heat being lost to or gained from the surroundings. How to reduce
and/or allow for these is covered in some practical books.
However, it is often not appreciated how inaccurate measurements of electrical power
can be. This can sometimes be due to the calibration of meters and the nature of the
supply: meters for dc are calibrated for smooth dc but sometimes they are used for
unsmoothed or partially-smoothed, rectified, full-wave ac, ie smoothed with one or
more capacitors and an inductor. It is not surprising that there is a considerable error
in the value of the power calculated from the product of two meter readings. While
the output of a regulated supply should produce no problems, the power available is
often too small to be useful.
Most analogue meters when purchased are accurate to about 2% of the full-scale
deflection. This implies the possibility of larger percentage errors in readings which
are well below full-scale reading. It must be remembered that, if two quantities are
multiplied as in a power calculation, the percentage errors are added.
Errors in power
calculations
Suppose the power of a heating coil is being measured with an ammeter and a
voltmeter, each with an accuracy ±2.5% full-scale deflection (fsd). The ammeter is
10 A fsd and the reading is 4 A: the uncertainty will be approximately 2.5 × 10/4 =
6.3%. The voltmeter is 100 V fsd and the reading is 12 V: the uncertainty will be
approximately 2.5 × 100/12 = 21%. The total uncertainty will be approximately
27%.
Corrections to
meters for
unsmoothed,
rectified ac
Most low-voltage power units provide a full-wave rectified output of the shape
shown below. Although this voltage always remains positive, it can hardly be
regarded as equivalent to the steady voltage produced by a car battery. The
undulatory nature of this supply is not significant in some school experiments
even where they are quantitative, eg, electrolysis.
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Voltage
Vp
Time
However, in experiments involving the measurement of power or the wattage of a
bulb, in heating experiments etc, the errors are often significant. The power, P,
developed in a load of resistance, R, is given by
V2
P= R
ie the value of V which must be measured is the root-mean-square value (as for
ac). For the wave shape above,
Vrms =
1
Vp
2
but a moving-coil meter measures V mean where
V mean =
2
π
Vp
and the difference amounts to an error of 10%. When the reading is squared this
error gives rise to a 20% error in the power.
Joulemeters
12.8.2
There have been complaints about the lack of accuracy of these, due at least partly
to a failure to understand their limitations. Recent models deal better with
unsmoothed or partially-smoothed dc than ammeters and voltmeters (see Heat
and power measurements above) but accuracy is still affected. Fixed voltage
joulemeters are likely to be more accurate than variable voltage ones. With the
latter, accuracy is affected if extreme values are chosen: high voltage with low
current or low voltage with high current. Users are advised to consult manufacturers’ instructions and keep well within the ranges quoted.
Cooling curves
is classified as harmful by skin absorption or by inhalation of the
m Naphthalene
vapour. It should not be used for class investigations of cooling curves in the open
laboratory; a fume cupboard must be used for work on this scale.
Good alternatives are hexadecan-1-ol (cetyl alcohol), octadecan-1-ol, hexadecanoic acid
(palmitic acid) and octadecanoic acid (stearic acid). Smaller quantities than were often
used with naphthalene in the past will suffice but a pure grade is necessary to obtain
clear, three-part curves. Ethanamide is another alternative but it must be dried in an
oven before use. Schools sometimes use paraffin wax or even candle wax but these will
not give clear, three-part curves.
12.8.3
Gas laws
Experiments to verify the relationships between pressure, temperature and volume of
a fixed mass of gas are less popular than they were, largely due to changes of
emphasis in the curriculum. Some of the apparatus uses mercury but this presents no
serious risk if handled carefully: the apparatus must be properly maintained and kept
stored over plastic boxes or other vessels to contain the mercury should there be a
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breakage; open tubes containing mercury should be stoppered when not in use; see
section 12.13 (Mercury).
Boyles’s Law, Bourdon
Gauge type
Boyle’s Law
Boyles’s Law, Constant volume
gas thermometer using mercury
Charles’s Law
There are two types of apparatus available from the normal school suppliers: both
apply pressure to air in a graduated tube, the first with an adjustable column of
mercury and the second with a car-tyre footpump, the pressure being measured
with a Bourdon gauge which gives a direct reading on a dial. (The setting of this
gauge assumes a standard sea-level value for atmospheric pressure; the supplier
should be informed if a model is required to operate at a high altitude.)
The foot pump-Bourdon gauge model helps to establish the principle of Boyle’s
Law more easily. A coloured oil to refill it in case of accidents can be obtained from
the supplier but any light engine oil would serve. (Two-stroke oil is often of a
suitable colour.)
The mercury apparatus is more accurate. There are dual-purpose models which
can be converted to constant-volume gas thermometers.
Constant volume This is sometimes called Jolly’s Air Thermometer. Models are available from the
gas thermometer normal laboratory supplier, in one case combined with a Boyle’s Law apparatus.
All use mercury. It is important that the gas is dry.
Constant
pressure gas
thermometer
m
Models are commercially available but it is simple to make suitable apparatus:
dry air is trapped with a thread of mercury or oil in a tube which is sealed at one
end. It is important that the air is dry and so a thread of concentrated sulfuric
acid (CORROSIVE) is sometimes used.
The acid is dangerous as its presence is easily forgotten. If it is used for this
purpose, the tubes should be stored in a closed container with silica gel and
suitably labelled.
12.9
High voltages
Electricity supplies with voltages greater than 28 V ac / 40 V dc and capable of delivering more than 5 mA are regarded as potentially dangerous in schools. The most
common instance of a high voltage is, of course, the 240 V mains required as a power
supply for many items of equipment. This is covered in section 6 (Mains Electricity).
However, there are other situations in which high voltages are met.
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Electrostatic generators
Electrostatic generators produce a lot of excitement and, apparently, a little hysteria.
Van de Graaff generators and Wimshurst machines produce high voltages but their
powers are too low for any X-rays produced to be hazardous.
Avoiding harmful electric shocks
Electrostatic generators used in schools (eg, Van de Graaff generators, Wimshurst
machines) do not cause harmful electric shocks because the small electric charges
produced lead to currents of well below 5 mA. Only equipment, specifically sold for use
in schools, should be used.
No attempt should be made to increase the capacitance of the charged system by
modifying the apparatus supplied because this could cause discharge currents to reach
hazardous levels.
capacitors, eg, d-i-y Leyden jars,
m Additional
Graaff generators or Wimshurst machines.
must not be connected to Van de
Electromagnetic interference
Sparks from electrostatic machines of all types are prodigious sources of electromagnetic interference, which may adversely affect the operation of nearby electronic
equipment. Indeed prolonged use might contravene the Electromagnetic Compatibility
Regulations.
Most importantly, anyone fitted with medical electronic equipment (eg, any type of
hearing aid or pacemaker) must keep well away from a working electrostatic generator because the electromagnetic interference may cause these devices to malfunction.
In practice, we recommend that, while viewing such a demonstration, a pupil wearing
such equipment should keep a minimum of 6 metres from a working generator. Very
occasionally, a pupil with a particular medical condition (eg, fitted with a cochlear
implant) may have received specific safety advice relating to the use of electrostatic
generators; this advice must be heeded.
It is wise to warn any computer users in the vicinity that a generator is to be used at a
certain time and that their computer data may be corrupted as a result. Nearby computers, interactive whiteboards or similar electronic equipment (eg, a data projector)
should be switched off and the generator operated well away from them.
Charging a pupil with a Van de Graaff generator
Many teachers use the Van de Graaff generator to illustrate the repulsion between
‘like’ electric charges by charging a child, to make his or her hair stand on end. This
activity is very safe, provided the guidance below is followed.
Safety when charging pupils
Only a Van de Graaff generator should be used for this demonstration; Wimshurst
machines must not be used. Personal electronic equipment, such as mobile phones,
personal organisers and wristwatches should be kept at least 2 metres away.
As a precaution, anyone with a known heart condition should not be accepted as a
volunteer for this activity.
Only one child at a time should be insulated from the ground and charged. When two
children are insulated and charged together, the shock experienced on discharge may
be sufficient to frighten the child receiving it. No children should be persuaded to take
part unless they genuinely wish to do so.
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Procedure when charging pupils
It is best to select a child with moderately short, dry hair. Make sure the dome or ‘top
conductor’ of the Van de Graaff generator is completely discharged. Stand the child on
a sturdy insulator (eg, a solid slab of expanded polystyrene or a child’s plastic stepstool). The child places both hands on the dome. The generator is then operated to
charge the pupil until his or her hair stands on end; shaking the head may assist in
this process. The use of a mirror or perhaps a camera may make the experience more
memorable for the volunteer. The charge tends to leak away from the extended hair
and the maximum potential reached is probably less than that on the generator when
running normally. Once the effect has been observed for a suitable time, operation of
the generator should be stopped.
The child may now let go of the dome and bring a finger towards an earthed point. He
or she will feel an electric shock, similar to that experienced on leaving a charged
motor vehicle or touching a metal object after walking across a synthetic carpet. It is
possible for the charged pupil to share the discharge experience with a chain of pupils
holding hands and standing on the floor. The charged pupil reaches out a finger to the
pupil on the end of the chain and everyone in the chain may feel a small electric shock.
Less-adventurous teachers may wish to avoid any shocks by simply passing the
charged child a wooden metre rule. The charge leaks away via the wood with no sensation at all.
Maintenance of Van de Graaff generators
A Van de Graaff generator which is properly set up and adjusted may work well one
week but very poorly the next. There are two factors which can cause this result.
•
The presence of dust or dirt on the top conductor and on the supporting pillar.
• Dampness which particularly affects the insulating properties of the belt.
Keeping the
generator clean
Consequently, it is vital to keep a generator clean by covering it with a suitable
polythene bag between each use. However, this alone may not be sufficient
because any surface carrying an electric charge will attract dust and dirt from the
air. Before putting the machine away in its polythene bag, it is sensible to lift off
the top conductor (usually an incomplete sphere) to clean it and the structure
which supports the top roller, collector and conductor. This is done as follows.
i)
Fold two or three soft tissues into a wad.
ii) Moisten the wad with a suitable solvent, eg, isopropyl alcohol, Volasil 244 or
Swan lighter fluid.
iii) Wipe this over the outside of the top conductor, the surfaces of the support
and both sides of the belt.
Keeping the
generator dry
If a cleaned generator still does not work, the problem may be dampness. If this is
anticipated, the generator can be placed in a warm cupboard or location overnight
before it is required. If the generator is needed immediately, a hair drier can be
used as follows.
a)
Set up the generator and start it running with a spark gap of 5 to 10 mm.
b)
Play hot air from the hair drier onto the belt, pillar and top conductor for a
total of about three minutes or until sparks are observed.
c)
If there is still no sign of life, try charging up a polythene rod and holding it
near to the belt until sparking begins.
d)
Increase the spark gap to the normal working maximum; if the sparks do not
jump the gap intermittently, try further hot air.
If the generator makes a crackling noise but no visible sparks, there are probably
some dust particles on the top conductor which are causing ‘corona discharge’.
These can be removed by repeating the cleaning process.
Mainly Physics
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© CLEAPSS 2009
Corona discharge can also occur when the generator has been in use for many
hours and the top conductor has become roughened by repeated sparking at the
same place. Since the top conductor is usually made of spun aluminium, any
rough patches should be removed by rubbing the area with wet-and-dry abrasive
paper (start with 240 grade and progress to higher numbers to at least 600, all
used wet). Finally, use a metal polish, such as Silvo or Duraglit.
A generator which has a low-voltage motor in the base to drive the belt has been
known to fail because of faults in the transformer / rectifer system powering the
motor. This has been known to happen when the generator is run for a long time
during an open day. School Van de Graaff generators are not designed for
continuous running.
12.9.2
EHT units
These give up to 5 or 6 kV. Those supplied for schools have their output currents
limited to 5 mA or less and so are completely safe. Any EHT supply without this
restriction on current should never be used.
12.9.3
HT units
Schools use HT units which have outputs greater than 40 V dc and are capable of
supplying currents greater than 5 mA. They are necessary for electrophoresis (see
section 11.1.7) and for some discharge tubes, ie, some Teltron tubes which require a
current larger than an EHT unit can supply; see section 12.6 (Electron beam and
similar tubes). They can be used for Millikan’s oil-drop experiment.
units should be handled only by staff and supervised sixth form students and then
m HT
with extreme care. Leads fitted with 4 mm plugs having sprung shrouds must be used
and care must be taken with other devices in the circuit.
If units with HT outputs are used with younger pupils for their low-voltage outputs,
their HT outputs must be disconnected first, eg, by removing the appropriate fuse. If
the fuse is subsequently replaced, a large warning label must be fixed onto the unit.
12.9.4
Induction coils
Induction coils produce voltages well in excess of 5 kV, usually at powers sufficient to
constitute a hazard, if used to produce discharges in evacuated apparatus. They
should not be used for this purpose. School induction coils supplied by established
educational suppliers during recent years cannot produce a current above 5 mA and
are electrically safe. An older induction coil, or one from another source, is potentially
hazardous and must not be used.
12.9.5
Demountable transformers
These should be handled by pupils only if they meet the normal earthing and insulation requirements for mains-operated equipment as detailed in section 6 (Mains electricity) and if there is no risk of any secondary coil producing over 28 V ac at 5 mA or
more. During class activities with coils and C-cores, care should be taken to minimise
the risk that hazardous voltages may be generated.
12.9.6
Power line (National Grid) simulation
A simulation of the efficient transmission of electricity along power lines, as used in
the National Grid, is a popular and important demonstration in schools. Low-voltage
ac from a power pack is sent along a pair of model power lines made from suitable
resistance wire.1
1
Typically two 1 metre lengths of 28 swg wire made from a copper-nickel alloy known as Eureka or Constantan.
© CLEAPSS 2009
1233
Mainly Physics
At the far end, there is insufficient power available to light a bulb brightly, due to heat
produced in the lines. If the low voltage is transformed up, transmitted along the same
power lines and transformed down at the far end, the same bulb will light brightly.
This is because the lines carry a lower current, reducing the heating effect.
have been reported where school staff received electric shocks by touching
m Incidents
uninsulated model power lines during this demonstration. Careful use of one of the
control measures listed below has avoided such incidents in recent years.
Alternative
precautions for
the power lines
experiment
1. The voltage on an open (uninsulated) power line must not exceed 28 V ac.
OR
2. All conductors and terminations operating above 28 V ac must be fully insulated so
that they cannot be touched.
OR
3. The whole demonstration must be completely enclosed by insulating material, such
as a large polycarbonate case, so that all conductors and terminations operating
above 28 V ac cannot be touched.
Power line apparatus may be purchased ready-made from science equipment
suppliers. CLEAPSS previously published information on the construction of both low
and high-voltage systems. However we believe that only a few schools built their own
systems because the process proved to be time-consuming and nearly as expensive as
purchasing a ready-made system. Please contact our Helpline if you are considering
making your own system.
Low-voltage power lines
Some schools choose a low-voltage version (which uses precaution 1 above). The
arrangement is based a Nuffield design. If a general-purpose, low-voltage power
supply is used then the voltage control must be locked so that it cannot supply more
than 3 V ac. A fixed, low-voltage, 3 V ac power supply (often know as a Westminster
Power Supply) is preferable. This ensures that the voltage on the open power line
cannot exceed 28 V ac. Unfortunately, the transformers in a low-voltage system tend
to suffer greater power loss. This means that the differences in power line
transmission efficiency are usually less obvious than with a high-voltage system.
This ready-made, low-voltage system works
from a 3 V ac supply. It uses 2.5 V lamps
and two transformer units. The open power
lines may either be operated at 3 V or about
22 V ac so that transmission efficiency may
be compared.
It is supplied by Griffin.
www.griffin-education.co.uk
High-voltage insulated power lines
Most schools choose a high-voltage, insulated version (which uses precaution 2 above)
based on a CLEAPSS design. This works at a higher power level and generally gives
very much more convincing results than the low-voltage version because a lower
proportion of the electrical energy is lost in the transformers. Insulated boxes with
transparent lids allow inspection of the transformers and protect users from electric
shock. The power lines are covered with insulating sleeving.
Mainly Physics
1234
© CLEAPSS 2009
This ready-made, high-voltage system
works from a 12 V ac supply. It comprises
two enclosed transformers and two 12 V
lamps. The insulated power lines may be
operated at either 12 V or about 240 V ac
so that transmission efficiency may be
compared.
Manufactured by Electrosound.
www.electrosound.co.uk
12.9.7
Household power circuit simulation
Teachers sometimes wish to demonstrate a working model of a domestic ring main.
Instead of mains power, low-voltage lamps, etc, are operated from a low-voltage
laboratory power pack. In order to eliminate the risk of deliberate or accidental
connection of such systems to mains electricity, standard mains plugs must never be
used. We advise the use of ‘non-standard’ plugs and sockets. These are available from
suppliers such as Farnell and RS Components.
m
When selecting plugs and sockets for low-voltage work, make certain that the
plug type is not in use for mains electricity connections anywhere in the school.
Examples of non-standard fittings that may be used for household power circuit
simulations are shown below.
T-shaped earth pin plug and
socket manufactured by MK
(with product code).
647 WHI
K1257 WHI
Round earth pin plug and
socket
manufactured
by
MEM (with product code).
F3368
X120MG
Please note - pages 1235 to 1236 have now been deleted. In order to maintain the
existing page numbering, the text resumes on page 1237.
© CLEAPSS 2008
12.10
1237
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Ionising radiations
This has now been deleted because more up-to-date information can be found in
CLEAPSS guide L93 Ionising radiations and radioactive substances.
12.10.1 Category C work
This has now been deleted because more up-to-date information can be found in
CLEAPSS guide L93 Ionising radiations and radioactive substances.
12.10.2 More details on Category C work
This has now been deleted because more up-to-date information can be found in
CLEAPSS guide L93 Ionising radiations and radioactive substances.
12.10.3 Category B work with open sources
This has now been deleted because more up-to-date information can be found in
CLEAPSS guide L93 Ionising radiations and radioactive substances.
12.10.4 Electron beam tubes etc
See section 12.6 (Electron Beam and similar tubes) and also see the information in
guidance leaflet PS 76 Electron-beam tubes: an introduction.
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1238
© CLEAPSS 2008
12.10.5 Apparatus for work with radioactive substances
Wire mesh
welded to cap
Nickelplated
brass
Handling tools
Sprung ring
Active foil
disc etc
Lead
cylinder
Tube
Source in Araldite
Source in
Araldite
Perspex square
Since one of the most effective ways of reducing the rate at which radiation is
given to a user is to increase the distance between the person and the source, all
radioactive sources should be handled with a proper tool. This must allow the
source to be held securely and manipulated as required by the work to be done.
For the sealed sources used under Category C, of maximum activity 370 kBq, the
tool should allow the fingers to be at least 10 cm from the source.
Sensors
The sensors commonly used by schools and FE establishments include: a GeigerMüller tube, sensitive to β- and γ-radiation but only weakly sensitive to α-radiation;
solid-state detectors for α-radiation; ionisation chambers sensitive to α- and βradiations; spark detectors sensitive to α-radiation.
Geiger-Müller
tube
The GM tube most commonly used is the ZP1481, formerly MX168; it is halogenquenched and has a thin end widow. It needs a supply of about 400 V but this is
normally provided by the scaler or ratemeter to which its output is fed. The
window is protected by a plastic cap which has a grid over the window; it should
be kept on whenever possible, At one time a GM tube with a thinner window
intended for α-particle detection was sold (MX168/01) but this was unsatisfactory.
Also sold to schools in the past were hollow GM tubes to take liquids. However,
because they are expensive and because few establishments use them, they are
not now available from school suppliers.
Solid-state αradiation
detector
A SS detector is useful for counting α-particles. It is damaged by mechanical
shock and is also sensitive to light and UV radiation; it is affected by temperature
and humidity. It needs to be handled and used with care and stored in a sealed
container in a place not subject to extremes of temperature.
Ionisation
chamber
Usually this takes the form of a cylindrical container which has a well-insulated
central electrode, drilled with a 4 mm socket into which can be plugged sources etc
and terminating outside in a UHF connector; there will be a 4 mm terminal/socket
connected to the wall of the container. Some chambers are supplied with extension
electrodes which can be plugged in and also mounted pieces of zinc for photoelectric investigations. The container should have a lid and be fitted with inlet
and outlet tubes for connection to a thoron generator. An alternative gauze-fitted
lid permits investigations on the range of α-particles.
Ionisation chambers need to be used with a sensitive current-measuring device
such as a dc amplifier or a picoammeter; see section 12.7 (Electronic meters).
Spark counter
This is useful for demonstrating the presence of α-particles and measuring their
range in air. It consists of two parallel conductors, usually one a metal strip or
wire, the other a gauze mounted just above it on an insulated box containing a
high voltage capacitor connecting the conductors. The counter is connected to an
EHT supply which is adjusted so that sparks occur if a source is brought close.
© CLEAPSS 2008
1239
Mainly Physics
Measuring instruments
The GM tube and solid-state α-detectors are used with scalers or ratemeters and
ionisation chambers with an EHT or HT supply and a nanoammeter. A spark counter
requires an EHT supply but usually no other equipment. Interfaces enabling the
output from GM tubes etc to be monitored by computers are becoming available on the
education market.
Table 12.7 indicates the equipment needed for a range of typical investigations.
Scalers
Scalers count pulses. Those available from education suppliers are suitable for use
with a halogen-quenched Geiger-Müller tube for which they provide the
appropriate HT supply. All are suitable for use with solid-state detectors but
sometimes require extra external amplifiers for which they have power outputs.
All of the scalers available from education suppliers have timer functions for
dynamics experiments; see section 12.4.2 (Timing methods) above. Some also have
ratemeter and frequency meter functions. While it is an economy to have several
functions served by one instrument, they can justify the increase in price only if
used. Also, the more functions an instrument can perform, the more complicated
its controls must be, which may cause problems for a busy teacher who uses it
only occasionally. Further, to see the same complicated-looking box used in
different ways may confuse pupils.
A few scaler-timers have a facility enabling GM tube and solid-state detector
output pulses to be counted automatically over preset periods of time (eg, 1, 10 or
100 seconds). This facility is useful but may hide what is happening from pupils.
Ratemeters
Used with a GM tube, these give a direct reading, analogue or digital, of counts
per second. Some are designed to work with a solid-state detector and have
suitable power outputs for the extra preamplifier needed. Some scaler-timers also
have ratemeter functions.
Some ratemeters are designed for monitoring, for example, for checking contamination in open source work, and so are portable, powered by batteries.
Scaler or
ratemeter?
Measurements which can be made using a scaler can also be made with a ratemeter although, for α measurements, it must be able to accept pulses from a solidstate detector with a pre-amplifier, ie, must have a suitable input socket, usually
a PET coaxial socket, and, preferably, a power output for the pre-amplifier.
Similarly, a ratemeter can nearly always be replaced with a scaler.
To obtain a count rate with a scaler, the count has to be measured over a period of
time, from 10 or 20 seconds up to two or three minutes, and the count divided by
the time. The timing is repeated to give a measure of the random variation in
readings. The great advantage of a ratemeter with an analogue display (ie, needle
over a dial) is that changes in rate are clearly visible. These are less easy to see
with the digital displays of scaler-timers fitted with automatic timing to give a
ratemeter facility. However, ratemeters with analogue displays have a number of
minor disadvantages as listed below.
a)
b)
c)
The appropriate count rate range must be set with a switch and the corresponding scale on the dial used.
The appropriate time constant must be set with a switch. In effect, this
chooses the period of time over which the random variation in count rate is
averaged to give a more or less steady reading.
At very low count rates, the dial reading is so unsteady as to be unreadable.
Low count rates can be measured only by timing the pulses of sound emitted
by the loudspeakers with which ratemeters are fitted.
Although it is sometimes convenient to have a direct reading of count rate,
perhaps to indicate the presence of a radioactive isotope or qualitative changes in
incident radiation produced by a magnetic field, a ratemeter should usually be
purchased only as a ‘second instrument’, after a scaler whose use makes clearer
the concept of count rate and of the random nature of radioactive decay. However,
Mainly Physics
1240
© CLEAPSS 2008
the same ideas can be developed by timing with a stopwatch the audible output
(clicks) from a ratemeter counting at a low rate. The popularity in the past of the
scaler compared with a ratemeter is due to its usual combination with a timer.
Since timing is now often done using a computer interface, the relative usefulness
of scalers and ratemeters should be reassessed.
Table 12.7
Instruments for investigations with ionising radiations
Measurement
Radiation
1st choice*
2nd choice
α
β
scaler + ssd
scaler + GMt
ratemeter + ssd
ratemeter + GMt
α
ratemeter + ssd
β
scaler + GMt
scaler + ssd, spark
counter, ionisation
chamber with gauze
window.
ratemeter + GMt
Number of ions produced by an
α-particle
α
ionisation chamber + ionisation chamber +
picoammeter
dc amplifier
Inverse square law for γ-rays
γ
Deflection by magnetic fields
α
β
scaler + GMt,
(preferably a γ-GMt)
ratemeter + ssd
ratemeter + GMt
ratemeter + GMt,
(preferably a γ-GMt)
scaler + ssd
scaler + GMt
Penetrating power
α
β
γ
Exponential decay and half-life of
thoron
Exponential decay and half-life of
protactinium
α
scaler + ssd
scaler + GMt
scaler + GMt
(preferably a γ-GMt)
ionisation chamber +
picoammeter
ratemeter sensor +
computer
ratemeter + ssd
ratemeter + GMt
ratemeter + GMt
(preferably a γ-GMt)
ionisation chamber +
dc amplifier
scaler + GMt or
ratemeter + GMt
Counting particles from a source to
develop the concept of count rate
and to show the random nature of
emission
Range of particles in air
β
* Which is considered ‘first choice’ is very much a matter of personal preference
GMt: Geiger-Müller tube
ssd: solid-state detector
Other factors to
be considered
before purchase
of scalers and
ratemeters
The range of functions to be considered is shown in Table 12.7. Other factors are:
the visibility of the display, now usually liquid crystal (the instruments are often
used for demonstrations so that size of digits is important); the number of digits
(too many can be confusing but so can having to switch ranges); ease of use
(instructions printed on the instrument, clear grouping and labelling of sockets
and switches). One company sells this equipment in modular form which aids
pupil understanding of how it functions.
DC amplifier/
electrometer,
nanoammeter
Combined dc amplifiers/electrometers, used in the past for work with ionisation
chambers, were expensive and difficult to use. Picoammeters reading to 10-11 or
10-12 A are cheaper and easier to use. Some meters sold as nanoammeters may be
suitable; see section 12.7.1 (The different meters described) above.
12.10.6 Units and definitions
This has now been deleted because more up-to-date information can be found in
CLEAPSS guide L93 Ionising radiations and radioactive substances.
© CLEAPSS 2008
12.11
1241
Mainly Physics
Kinetic theory models
These models are intended to simulate the motion of gas molecules, that is a random
motion of small particles, displaying pressure by the particles hitting against a solid
surface. It should be possible to increase the average velocity to simulate increase in
temperature and to show that an increase in applied pressure reduces the volume.
There are three kinds: commercial models in which motion is produced by a vibrating
piston at the bottom of a tube; a d-i-y model using a small loudspeaker fitted into a
tube and energised with a signal generator set at a low frequency; a d-i-y two-dimensional model for an overhead projector, energised with a electrostatic generator, eg, a
Van de Graaff generator.
Mechanical models to demonstrate kinetic theory are not popular with teachers for
three reasons: they are noisy; liable to go wrong, perhaps with the balls spilling on the
floor; liable to confuse pupils who mix up the theoretical kinetic theory model with the
mechanical models they have been shown. Some teachers prefer computer simulations1 which are particularly effective if a large-screen monitor is available.
These consist of a transparent tube, square or circular in cross section, with a viCommercial
brating piston at the bottom end. This piston keeps small spheres, eg, 3 mm
models
Loudspeaker
model
phosphor-bronze ball bearings, in motion. This will keep another piston floating;
this is usually of card or expanded polystyrene. By loading this piston with extra
card disks, etc, it is possible to simulate Boyle’s Law: the greater the weight of the
piston, the smaller the volume occupied by the moving balls. The piston is powered by a small electric motor which can be speeded up by increasing the voltage,
simulating the increase in average kinetic energy of the balls with temperature.
In order to keep the cost down, the motor tends to be an inexpensive type,
insufficiently robust to withstand careless treatment. Its life can be extended by
the regular lubrication of moving metal parts and care when starting. If the motor
stalls, the supply must be switched off.
A small loudspeaker is clamped horizontally and has a transparent tube sealed
above it, with Plasticine, Blu-Tac etc. The tube can be rigid Perspex or made out of
a thin sheet of transparent plastic, eg, an OHP sheet, bent round into a tube. The
loud speaker is energised by the low impedance output of a signal generator. The
‘molecules’ need to be light; puffed wheat is worth trying. They will support a
floating piston; see the diagram.
Kinetic theory
models
Clamp not closed
Wire
Card
Expanded
polystyrene
piston
To Van de Graaff
Petri dish
Tube
made of
sheet
plastic
Loudspeaker
Diaphragm
Crank on small
electric motor
Typical commercial model
2-dimensional
OHP model
12.12
To signal generator
Cut-outs with teeth covered with foil
D-I-Y models
Two zigzag cut-outs of wood or expanded polystyrene have their ‘zigzags’ covered
with kitchen foil. They are put in a large Petri dish placed on an overhead
projector and connected to a Van de Graaff generator. Silver ball cake decorations
are poured between the cut-outs.
Lasers
A low-power, continuous-wave, helium-neon laser is useful for teaching wave optics
because it produces a beam of light which is:
highly monochromatic (very narrow spread of wavelengths) and coherent (the
same phase) both across the beam and with time;
1
For example, Particles and Diffusion or Kinetic Theory and the Gas Laws from AVP.
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1242
© CLEAPSS 2008
This page has been deliberately left blank. Pages 1243 to page 1252 have been
deleted. The text resumes from the next page which is page 1253.
© CLEAPSS 1992
1253
Mainly Physics
of high intensity;
of small divergence (typically 1 mm in diameter when it leaves the laser, 5 mm in
diameter 4 m away).
of its intensity, care must be taken to prevent a beam from a laser falling on
m Because
the eye directly or by reflection, as it could damage the retina. In fact, it is now known
that eyes could be damaged by the beam from the low power Class II lasers used in
schools only if a deliberate attempt were made to stare at the laser along the beam or
to concentrate the beam from a IIIa laser with an instrument; the normal avoidance
mechanism of the body would prevent damage in other cases. Nevertheless, it is good
practice to take precautions to avoid a beam falling on the eye.
Lasers should be positioned so that beams cannot fall on the eyes of those present, ie,
are directed away from spectators. Ancillary optical equipment must be arranged so
that reflected beams cannot reach eyes.
Ten years ago, the lasers affordable by schools would work for only a few years and
then only if run periodically. Those currently available have longer lives and do not
require this.
DES
Administrative
Memorandum
7/70
This advises on the use of lasers in schools and FE colleges. It confines use at
school level to teacher demonstration. Safety rules include the avoidance of direct
viewing; screening pupils who should be at least 1 m away; keeping background
illumination as high as possible (so that eye pupils are as small as possible);
displaying warning notices; keeping lasers secure from unauthorised use or theft;
use of goggles by the demonstrator. Because of the more recent realisation that
the lasers used in schools and colleges will not harm an eye by a beam
accidentally falling on it, these rules are not strictly necessary but following them
is good training. However, goggles are expensive, impracticable and do not reduce
hazard as they make it harder for the demonstrator to see the beam, stray
reflections etc.
What to look for
when buying
For school or college science, a Class II helium-neon laser with an output of nearly
1 mW is advisable. While a Class IIIa laser can be used safely, more powerful
lasers should not be used. The laser should be fitted with a key switch to control
its mains supply to prevent unauthorised use. It is not necessary for the beam to
be plane-polarised; if this is required, it can achieved with a Polaroid sheet. While
it is interesting to be able to send messages along a laser beam, it is a luxury to
buy a laser with built-in modulation; external methods may be used.
12.12.1 Use of lasers
A laser is best arranged for demonstration pointing away from those observing, at an
angle to the line of sight so that the demonstrator does not block their view. It should
be placed on a board, big enough to take accessories, which can be tilted upwards at
one end to point to a raised screen. This screen, which can be a piece of drawing paper
on a board, should usually be 2 to 4 m away.
Kits of accessories are available from suppliers but, while adjustable slits etc are very
convenient, they are expensive and can be improvised. Slides for diffraction patterns
and holograms are less expensive and worth buying.
It is important that adjustments can be made easily to the relative positions of laser
and accessories so that they can be lined up quickly and accurately. Normal holders
are too high for most laser beams but the laser can be raised on blocks of wood, books
etc.
Adjustable slit
This can be improvised using two razor blades. These can be held on a magnet
which allows easy adjustment, stuck to a flat metal or plastic strip with thick
grease or over a circular hole in a piece of card with Plasticine.
Mainly Physics
Table 12.8
Effect
Width of slits,
diameters of pinholes
etc
1254
© CLEAPSS 1992
Basic demonstrations of wave optics with a laser
Apparatus*
Arrangement
Slit, pin etc
Holders
+10 D lens
u
Note
A sharp image is
formed on the screen
and then:
v
(Useful if λ is to be
determined )
To obtain a wider
beam
(For slits more than
1 mm apart etc)
Young’s fringes
Diffraction at a single
slit, pinhole etc
width = u/v × image
width
-5 D lens
+10 D lens
Holders
10 cm
Slit(s) on slide, etc
Holder
Beam widening
arrangement if
needed
Multiple slits
Slits on slide
Holder
Beam widening
arrangement if
needed (above)
Diffraction grating
Diffraction grating
Holder
10 cm
Approx 3 m
Adjust the position of
the 10 cm lens to
obtain a sharp spot on
the screen before slits
etc are placed in
position.
2-4m
See text for preparing
slit, pinhole etc.
Approx 3 m
2-4m
Arrange grating lines
to be horizontal, so
that diffracted beams
are not directed at
spectators.
Wall and
ceiling
25 cm
Glass millimetre scale Millimetre scale
as a diffraction grating Adjustable table
Beam widening
arrangement if
needed (above)
P
Scale
Thin film interference
Thin film (see text)
+10 D, -5 D lenses
Holder
3 m at least
D
Enables wavelength to
be determined. Use
whole length of lab.
Beam glances along
scale. λ =
O
d × OP × NP /2D2
N
d is separation of
rulings
Safety
screen
10 cm
Approx 1 m
50 cm +
*
A laser and a screen are needed in each case. λ is the wavelength of the light, 632.8 nm.
Double slits etc
These can be ruled with a knife or razor blade on a microscope slide coated with
Aquadag (colloidal carbon) or cut in thin card.
© CLEAPSS 2005
1255
Mainly Physics
Pinholes etc
These can be made in pieces of cooking foil. Objects, such as ball bearings can be
stuck on microscope slides.
Thin film
A suitable film is that between a microscope slide and a piece of good quality
glass, held together with elastic bands. For Newton’s rings, use a 0.1 D lens on a
microscope slide.
A laser beam can be made visible by blowing smoke or making dust in its path. Its
path through a tank of water can be shown by adding a little milk. The principle of
optical fibres can be shown by directing a laser beam down a flexible plastic tube
containing water to which a little milk has been added.
Some demonstrations with a laser are outlined in Table 12.8. For details and further
uses, consult one of two excellent booklets: A Manual of School Laser Demonstrations,
W K Mace, from Philip Harris or Experiments with the Griffin Laser Accessories Kit, G
E Foxcroft, from Griffin & George.
12.13
Mercury
main hazard
m The
metallic mercury.
is the inhalation of mercury vapour from uncovered surfaces of
However, with a few simple precautions, mercury can be used
safely. It must be realised that mercury vaporises slowly and so its vapour can be
removed as it forms by moderate ventilation.
Clearing up spills of mercury is covered in section 7.7 (Chemical spills). Risks in using
mercury and its compounds, with the precautions necessary, are outlined on the
Mercury Hazcard. This section of the Handbook considers the use of metallic mercury
in school physics but the advice will also be relevant to chemistry and biology if
mercury manometers are used.
While the main hazard is the inhalation of mercury vapour from uncovered surfaces of
metallic mercury, there is another hazard associated with metallic mercury, its
absorption through the skin. This can occur if droplets of mercury remain behind
finger nails or fall into a shoe. Therefore, the wearing of rubber gloves and hand
washing are recommended. Swallowing a few grams of metallic mercury is not a
serious hazard; nevertheless, should it occur, medical advice should be sought.
Mercury vapour
levels
Mercury was previously given an Occupational Exposure Standard of 0.05 mg m-3
long-term exposure limit, 0.15 mg m-3 short-term exposure limit. It has not yet
been assigned a Workplace Exposure Limit, so we will continue to refer to the
older standard for guidance.
CLEAPSS offers a mercury monitoring service and, if the general level detected in
a room is more than 0.01 mg m-3, ie, 1/5 of the OES, it recommends that steps are
taken to lower the level. Any traces of mercury must be removed or sealed over or
the ventilation improved until the level is substantially lowered.
CLEAPSS has, over the years, monitored the mercury vapour level in some 10 000
science rooms; only in about 1% of these rooms was the general level above
0.01 mg m-3.
Mercury
poisoning
In schools, cases of mercury poisoning from the inhalation of its vapour are very
rare and very mild Since the symptoms of chronic mercury poisoning are not
specific (fatigue, lethargy, headaches, nausea, diarrhoea, sore gums, muscular
tremor, intellectual deterioration, irritability, loss of confidence etc), it is very
difficult to identify without hospital tests.
In mild cases of poisoning, the only kind which could occur in schools, the mercury
is excreted rapidly so that removal of the patient from the vapour results in
complete recovery.
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12.13.1 Situations where care is needed
Mercury vapour can affect health if mercury surfaces of moderate area are exposed
over a period of time in a room with poor ventilation in which a person spends long
periods. The rate of evaporation increases with temperature.
Mercury in
apparatus etc
Vessels and apparatus should be sealed and steps taken to ensure adequate
ventilation before mercury is handled.
Mercury in
between floor
boards etc
Mercury can sometimes be found in cracks between floor boards or blocks, in
benches, in drawers and cupboards, in corners, in ducts, around the edges of plug
holes, in sink traps etc. Old buildings tend to have more of the places where
mercury can lurk than new ones. More is likely to have been spilt in physics
laboratories and preparation rooms than other science laboratories and, the older
the building, the more time there is for it to have been spilt. Attention should be
paid to old laboratories converted to classrooms or other rooms.
High
temperatures
The saturation vapour pressure of mercury increases rapidly with temperature.
Raising the ambient temperature of a room will increase the mercury vapour level
and a hot pipe or poorly insulated oven near traces of mercury can have a big
effect. Drops of mercury from a broken thermometer must not be swept under the
radiator!
Poor ventilation
Mercury evaporates slowly so that, with normal ventilation through gaps around
windows and doors, the concentration of vapour at face height will be low despite
the presence of mercury in cracks in the floor, etc. SSERC1 suggests that, before
the ventilation of a room is bad enough to allow a dangerous concentration of
mercury vapour to build up, body odour will make it unpleasant to teach in.
Nevertheless, if it is suspected that there are traces of mercury in a room, it must
be kept well ventilated.
Long periods of
exposure
A concentration encountered only a few hours a week may produce no ill effects;
breathed in for seven hours a day for five days a week it may be harmful. This
suggests that technicians are more at risk than teachers and pupils and special
attention should be paid to preparation rooms during monitoring.
12.13.2 Precautions when handling mercury
Monitoring and health surveillance
Monitoring with an ultraviolet absorption meter is necessary only in poorly-ventilated
rooms with places where mercury may be present: eg, cracks in floors and duct covers,
false floors under bench cupboards, platforms etc. It should be carried out if there has
been a history of carelessness with mercury or visible traces suggest that it is there.
In recently-built laboratories with crack-free floors, much can be achieved with a
thorough visual inspection.
If a person is suspected of having been exposed to high levels of mercury vapour, the
level of mercury in his/her body should be checked at a hospital.
Spills
Spills should be cleared up carefully: see section 7.7 (Chemical spills).
Detailed precautions
Stock control
1
Stocks of mercury and, where possible, apparatus containing mercury should be
kept locked up. A signing-out system may be thought advisable.
SSSERC Bulletin, April 1977; see also Borrows, T P, Precautions in the use of mercury, Education in Science, 72, April 1977,
p20.
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The gross weights (strictly masses) of bottles of mercury (and, perhaps, of
apparatus containing large quantities of mercury) should be recorded. This
enables thefts and unrecorded spills to be detected and the magnitude of spills to
be assessed. Weights should be recorded in the laboratory stock book as well as on
the bottle or apparatus. (Screw eyes inserted into the backs of some pieces of
apparatus will enable them to be weighed easily with a spring balance.)
Spills should be recorded in the laboratory accident book or safety file; entries
should include date, exact place (which part of the laboratory), extent of loss
(thermometers should be assumed to contain 5 g), action taken, by whom.
Periodic checks of the gross weights of stock bottles and apparatus should not take
long and will reveal any unrecorded losses of mercury. The spills record will
indicate any areas where mercury spills have occurred; several minor spills in one
area would justify careful inspection and perhaps monitoring of the vapour level.
Containers
Cases have been reported of 500 ml polythene bottles containing mercury splitting
along the seams. Bottles should be examined regularly for weak seams. 50 ml
bottles are less liable to develop weak seams. All bottles should stand in a dish.
Cleaning and
disposal
Schools should not attempt to clean mercury except to remove dust from its
surface by shaking it gently in a sealed flask with a crumpled length of Sellotape.
It may be dried by laying filter paper on its surface. Schools should keep a bottle
into which mercury collected up from spills can be stored before disposal through
a disposal contractor or scrap metal dealer. It is now difficult to have small
quantities of mercury purified.
Precautions
when in use
Mercury should never be heated in an open vessel or in any closed vessel or
apparatus which might break or leak except inside an efficient fume cupboard.
Preparing mercury by heating mercuric oxide should be carried out only in a fume
cupboard.
It is advisable to wear latex gloves when handling mercury to prevent traces
being trapped under finger nails. Polythene gloves increase the risk of apparatus
being dropped. If gloves are not worn, hands should be washed after handling
mercury, with a nail brush and attention paid to finger nails, particularly before
smoking.
Mercury should be kept away from metal objects including rings and other
jewellery; it readily amalgamates with many metals. A gold ring may be gently
heated over a Bunsen burner in a fume cupboard to drive off mercury. If
overheated, gold will discolour but this discoloration polishes off quickly in normal
wear.
Mercury should be handled over a large, deep tray with a smooth impervious
surface. Metal is not suitable and, if wood is used, it should be given several coats
of polyurethane varnish or paint or a replaceable lining of thin polythene sheet.
Mercury-in-glass For precautions when using these, see section 10.7 (Thermometers).
thermometers
Setting up a
barometer
This interesting and instructive demonstration is well worth doing. The room
should be well-ventilated and the demonstration should be carried out over a large
tray covered with 5 mm of water. Rubber gloves should be worn or hands washed
thoroughly afterwards with a nail brush and attention paid to finger nails.
The Nuffield-pattern trough (a scooped out wooden block) minimises the quantity
of mercury needed although, as it cannot take all the mercury in a barometer, it
makes dismantling more difficult.
Inexperienced teachers will need to be shown how to do this demonstration1 and
inexperienced technicians how to clear it up. To let the mercury out of a barometer
1
Some teachers may prefer a method attributed to E M Somekh. A section is cut from the top of a bung to form a rubber disc and
the cut surface of the disc is attached with adhesive to the bottom of the vessel acting as a trough. The trough is inverted and the
rubber disc pressed over the end of the barometer tube, filled proud. Both are then inverted together, an assistant pours a little
mercury into the bottom of the trough and then the tube is lifted off the rubber disc.
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when the trough is too small to take all the mercury, a finger should be inserted
over the end and the mercury released into a vessel with a narrow neck, eg, a
250 ml conical beaker, over the water-covered tray.
If the barometer tube, trough, etc are really clean beforehand, clearing them of
traces of mercury afterwards is much easier.
Made-up
barometers,
Boyle’s Law
apparatus,
eudiometers, gas
thermometers
It is important that, where possible, versions of apparatus which do not require
mercury or which require the minimum of mercury should be purchased.
Manometers
These should be stored and used as above. Again, exposed surfaces should be
covered with a centimetre of di-n-butyl phthalate (not if it can come into contact
with pvc tubing) or distilled water. Alternatively, arms should be stoppered or
covered with inverted test tubes when not in use.
Apparatus should be stored on a tray and used over a large tray covered with
5 mm of water. Exposed mercury surfaces could be covered with a centimetre of
di-n-butyl phthalate (not if it can come into contact with pvc tubing) or distilled
water; the disadvantage of water is that it is likely to evaporate during storage.
Whichever liquid is used, the hydrostatic pressure exerted by the layer may well
need to be taken into account in measurements. Alternatively, when the
apparatus is not in use, reservoirs and tubes should be stoppered or covered with
inverted test-tubes. The apparatus must be checked periodically to ensure rubber
tubing, taps, etc are in good condition. PVC tubing is likely to last longer than
rubber tubing but is less flexible; it is damaged by dibutyl phthalate.
With 8 ft water manometers available, it seems unnecessary for pupils to blow
into mercury manometers. If they do, the mercury surfaces must be covered and
at least 200 mm spare height allowed; pupils must be carefully supervised.
Electrical
These items include commutators, Barlow’s wheel and Faraday’s experiments
experiments and with wires rotating round magnets. None is necessary as all have reasonable
substitutes.
apparatus
Barlow’s wheel produces a high concentration of mercury vapour and should
never be used except on a tray in a fume cupboard with a face velocity of at least
0.3 ms-1. Alternatively, 1M sulfuric acid saturated with copper sulfate can be used.
The liquid tray should be lined with copper foil, which should be the positive
connection with the wheel negative. Although it is valuable to reverse polarity to
show opposite rotation, this should not be done for long. Any demonstration of
Faraday’s experiments should be over a tray inside an efficient fume cupboard;
they may be attempted with copper sulfate solution as above.
Apparatus should be cleared of mercury before storage; alternatively, it can be
sealed in a plastic box or a plastic bag.
Ovens and
incubators
These are easy to contaminate with mercury by breaking a thermometer in them.
Once contaminated, they are difficult to clear of all traces and their raised
temperature means that, close to them, the mercury vapour level will be high.
Particularly prone to thermometer breakage are combined incubator-ovens: a
100 °C thermometer is inserted to monitor incubator temperatures and bursts
when the oven switch is changed to produce temperatures above 100 °C.
Schools should not use mercury thermometers for ovens and incubators. They
should use alcohol thermometers for incubators and bimetallic dial thermometers
or electrical thermometers for ovens and combined incubator-ovens; see section
10.7 (Thermometers) or CLEAPSS Guide R157, Thermometers. Any locking device
on an oven thermostat control should be screwed up tightly to discourage
unauthorised alteration.
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Measuring the
diameter of a
capillary tube:
surface tension
To measure the diameter of a tube for a surface tension determination, break at
the meniscus position and measure the diameter with a travelling microscope. It
is more satisfactory not to use thick-walled capillary but to prepare the capillary
fresh for each experiment by drawing out a piece of small diameter tubing.
Measuring the
diameter of a
capillary tube:
viscosity
There is no satisfactory alternative to the mercury method. The tube should be
dipped into mercury in a test-tube and withdrawn with a finger over the end. The
test-tube and a small crucible for weighing can be kept in a sealed plastics
sandwich box which will also serve as the tray.
Amalgamation of It is better not to amalgamate. Zinc sulfate may be used instead of acid in the
zinc rods for cells Daniell cell and the rods removed from the Leclanché cells when not in use.
Floating metals
on mercury
Metals are sometimes floated on mercury as a density demonstration. It is safe to
use iron, steel or brass coated with lacquer but never aluminium which could
catch fire. Alternatively, Wood’s metal, heated on a water bath, may be used in
place of mercury.
Boys’ method for The radius of curvature of a lens may be measured by putting black paper behind
it when working in semi-darkness with an illuminated pin.
measuring R
Sinks
CLEAPSS monitoring for mercury vapour levels shows that, in a small proportion
of laboratories, significant levels of mercury vapour are found in sinks, particularly if these are covered.
If the level in a sink near the waste outlet is below 100 µg m-3 (or more for a
covered sink), this normally has no detectable effect on the level in the air around
the laboratory that teachers, technicians and pupils are actually breathing.
However, some will still wish to find and remove the mercury causing it.
The advice that follows is the best on offer at present for decontaminating sinks
without a major dismantling operation; it is not always successful.
The first priority is to ensure that the sink itself is free of mercury. Likely hidden
areas of contamination are around the rim where this adjoins the bench or around
(even underneath) the flange of the waste outlet. If any mercury is found it should
be removed using a pooter or copper/acid paste. See section 7.7.4.
Next, if there is a trap (one trap may serve several sinks), this should be removed
if possible. Place a bucket or bowl under the trap before removal to catch any spilt
waste. Plastics traps should be removable using the hands alone but, if the unions
have been over-tightened, tools may be used, care being taken not to crack the
plastic. Metal traps may well be corroded and in some cases may be impossible to
reassemble after dismantling. Excess water may be decanted from the trap which
should then be taken outside and the sludge likely to be found in it put into a
thick polythene bag. Mercury may well be visible in the sludge and gloves should,
of course, be worn for this operation.
The trap can then be rinsed thoroughly over an outside drain using water from a
hose. A large test-tube brush is helpful for scouring out the trap. When the trap
has been refitted to the sink, approximately 1 l of lime/sulfur mixed with hot
water to form a slurry of consistency similar to custard should be poured down the
waste outlet and left for as long as possible before the sink is used again
(preferably a week or more). Where several sinks feed one trap, slurry should be
poured down each one.
If the trap is not removable or there is no trap, the following procedure should be
adopted; it is only sometimes successful. First fill the sink and empty it several
times; the flood of water should flush out some light debris. At least 1 l of
lime/sulfur slurry should then be poured down the waste outlet and left for at
least a week. If the trap is large or pipe runs are long, use more slurry.
Carpets
Any carpet contaminated with mercury should be disposed of. Vacuum cleaners
should never be used on a carpet thought to be contaminated with mercury.
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Carrying mercury It is obvious that clearing mercury from inside a car is a very difficult procedure.
If mercury has to be carried in a car, a polythene bottle inside a Merck ‘Safepak’
in a car
should be used. Mercury-containing apparatus should always be transported
empty.
Vacuum cleaners Since normal vacuum cleaners become hopelessly contaminated if they are used to
suck up mercury, they should never be used. If one becomes contaminated, it must
be disposed of.
12.14
Oscilloscopes
12.14.1 Use
The following books have been found useful:
Title
Author/
Date/ISBN
Publisher
Note
Student Oscilloscope
Handbook
Unilab
Unilab
Describes the use through instructions for 20 typical
experiments.
Oscilloscopes
How to use them, how they
work
Ian Hickman,
1990, ISBN
0434908088
Well illustrated and very informative, on oscilloscopes and
Heinemann
their components. Not for beginners but very helpful to the
Newnes but
available from RS interested teacher who is not an expert in electronics.
Obtaining a trace
It is not possible to give general instructions to obtain a trace on the screen: the user
must refer to the oscilloscope handbook. However, if the handbook is missing, the
following procedure could be tried.
Turn the brightness and focus controls fully clockwise.
Set the shift controls near their mid positions.
Select a Y gain (or ‘sensitivity’ or ‘attenuation’) of about 1 V/div.
Select a time-base speed (or frequency) near the middle of the range available.
Switch on. (The mains switch may be on the back or combined with another
control.)
Allow the tube to warm up and, if no trace appears after two minutes, look for the
time-base trigger controls: try changing a switch from ‘ext’ to ‘auto’ or vice-versa.
If no trace appears, make adjustments to the shift controls and again change the
trigger controls. (A ‘beam finder’ or ‘trace locate’ button may help here.)
If still no trace has been obtained, try connecting a wire to the Y input socket (the
centre contact if it is a coaxial type) to act as an aerial and try increasing the Y
gain (0.1 V/div).
When a trace has been obtained, the brightness, focus and shift controls can be
used to obtain a bright, sharply-focused trace centred on the screen.
To measure potential difference and current - dc and ac
Potential
difference - dc
Obtain a horizontal trace on the screen. If the trace disappears in the absence of a
Y input signal, switch the time-base off and obtain a well-focused spot (not too
bright).
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Set the Y input switch to ‘gnd’ or ‘earth’. If there is no earthed condition on the
switch, set it to dc and short-circuit the input. Use the Y shift control to set the
trace on to a low graduation on the graticule.
Set the Y input switch to dc (or remove the short circuit) and connect the Y input
to the dc potential difference to be measured. If the trace deflection is too small or
too large for convenient measurement, adjust the Y gain to obtain a convenient
deflection and then check the setting with the input earthed.
'Internal earth connection'
0.1
pd
ac
dc
gnd
V/cm
The voltage is measured by counting the number of divisions through which the
trace has been deflected, y, and then this is multiplied by the gain setting, say
0.1 V/div.
ie,
pd = y × 0.1 volt
If the Y gain is continuously variable, or a variable control is not set to the calibrated position when the measurement is made, it is necessary to calibrate the
oscilloscope by measuring the voltage required to produce a convenient deflection.
This enables the sensitivity to be calculated in volts per division.
Potential
difference - ac
The oscilloscope is prepared as above except that the trace is set near the centre of
the screen. (NB The Y input switch is still set to dc.)
Signal
generator
~
A
0.2
ac
dc
gnd
B
V/cm
pd
C
Connect the Y input to the ac potential difference to be measured. The trace
should spread in the vertical direction. Select a Y gain which enables the spread
to be measured. Either the time-base can be switched off to give a vertical line or a
low time-base speed can be selected, so that the peaks of the waveform are close
together. Set the lower peaks to a graticule graduation and count the divisions up
to the upper peak level (y). The peak-to-peak value of the ac is then the product of
y and the gain setting (say 0.2 V/div)
peak-to-peak ac pd = y × 0.2 volt
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Care must be taken in assuming that the rms value is this peak value divided by
√2; the apparent rms value assumes a perfect sine-wave shape. In the case shown
in the figure below, the true value is lower; in other cases it could be higher.)
NB
The peak-to-peak reading is different from that which would be obtained
from an ac voltmeter which could give any one of the following: peak value;
mean rectified value; true root-mean square value; apparent root-mean-square
value.
Apparent rms value
0.7 x peak
Mean
rectified
value
Earthing
problems
Peak
to
peak
True rms value
The circuit above also illustrates a common problem in measuring with oscilloscopes: one terminal of this signal generator is earthed and one terminal of this
oscilloscope is also earthed. If an attempt is made to measure the voltage across
AB instead of BC, the two earth connections short-circuit resistor BC and so
change the circuit under test. The following methods may be used to overcome this
problem:
1.
measure VDC, then VAC and calculate VAB ( = VAC - VBC) but this only works if
VAB is in phase with VBC;
2.
isolate the signal generator from earth using a power-isolating transformer
between the generator and the mains; do not simply remove the earth
connection in the mains plug as this would be dangerous;
3.
isolate the circuit from earth using an audio-isolating transformer between
the generator and the circuit;
4.
use an oscilloscope with a differential-input amplifier (easy at low sensitivities but expensive at high sensitivities); very few oscilloscopes recommended
by CLEAPSS, ie mentioned in CLEAPSS Guide R31, Oscilloscopes for
Schools, have this feature.
None of these methods is universally applicable and in different demonstrations
and class exercises different solutions will be appropriate; however, method 3 is
often the easiest and cheapest to implement.
Current - dc and
ac
The oscilloscope is essentially a voltmeter but it can be used to measure a current
by measuring the potential difference across a resistor of known resistance. In
many circuits, a suitable resistance will be present anyway but, if a resistor has to
be introduced, it should be small compared with other circuit resistances. In the
15 kΩ
1 kΩ
Y
Input
~
Measuring collector current
Y
Input
~
1 kΩ
R
Measuring base current
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left circuit just above, the oscilloscope is being used to measure the total current
through the collector load: suppose the mean deflection measured is 4.5 divisions
at 2 V/div.
Mean pd = 4.5 × 2 = 9 V. Since resistance is 1 kΩ, mean current = 9 mA.
The alternating current component can be deduced by a similar method; it may be
convenient to remove the dc component by switching the Y input to ac.
In the right circuit, the base current of the transistor is to be measured and resistor R has been introduced to do this. The value of R is chosen to be small compared with the effective resistances but not so small that the voltage across it cannot be measured with the oscilloscope. R could be 50 Ω in this case, assuming an
effective base–emitter resistance of 10 kΩ and a signal–source resistance of 1 kΩ.
To display wave shapes
The shapes of the waveforms generated by a modern signal generator, sine, square or
triangular, can be demonstrated by connecting the signal to the Y input. The trace
must be ‘stabilised’; ie, made to appear stationary, and, since the procedure varies
from instrument to instrument, the oscilloscope handbook should be consulted.
In the absence of the handbook the following can be tried:
select a time-base speed which gives a recognisable trace even if it is not
stationary;
select ‘internal’ on the ‘trigger’ or ‘synch source’ switch;
select ‘automatic’ as the time-base mode or ‘normal’ if automatic is not
available;
if necessary, adjust ‘trigger level’ or ‘synch level’ controls for stability.
In teaching electronics, sound, oscillations and waves
Electronics
An oscilloscope, preferably dual trace, is useful for comparing the input waveform
with the output from an electronic circuit to illustrate amplification, pulseshaping, delay, etc.
Sound
It is often useful to feed the oscilloscope from a microphone receiving sounds from
musical instruments or a loudspeaker. In this case a screened lead should be used
between the microphone and the Y input socket (which should be a coaxial one).
Oscillations
The oscilloscope can be used with a suitable transducer (eg, Harris Electronic
Arm) to display and analyse the motion.
Waves
The oscilloscope forms a useful display whether the waves are sound (using a
microphone), radio or microwave (using a diode detector). Traces on the oscilloscope screen can be used as convenient analogues of wave motion during discussion or revision of basic concepts. Beats, standing waves and modulation can
all be illustrated.
NB
Students have been confused by this use of the oscilloscope and teachers
should emphasise that the trace is normally a plot of a voltage against time not
displacement against distance.
To measure time interval, frequency and phase shift
These measurements require a calibrated time-base and it is recommended that at
least one oscilloscope in the school should be of the type which is always calibrated, ie,
there is no continuously variable control of time-base speed. The most common
mistake made by users of the oscilloscope is to make measurements assuming a
calibrated setting only to find afterwards that it was offset from the calibrated
condition.
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The oscilloscope is very well adapted to the measurement of time intervals which
are too short to be measured by means of a stop watch or even a millisecond timer,
particularly where that time interval occurs repetitively. For example, the figure
shows how the pulse length produced by a monostable circuit (eg, Nuffield
Advanced Physics Electronics Kit module) can be checked. The time-base is
triggered by the start of the pulse and a mode switch would be set to ‘normal’ or
‘triggered’ so that the resting position of the beam is visible as a dot at the origin.
This can then be set onto a vertical line on the graticule so that the number of
divisions, n, corresponding to the pulse length can be counted. For a time-base
speed of 1 ms/div,
the pulse duration = n × 1 ms.
Checking the
pulse from a
monostable
circuit
Monostable
2 V/div
I/P
1 ms/div
O/P
Y I/P
Trigger
normal
+ve
If the time-base cannot be operated in the ‘normal triggered’ mode, it may be
necessary to select a time-base speed such that two pulses are displayed, the first
having an uncertain start time. Measurements would then be made on the second
pulse.
Frequency
measurements
These may be made in the same way as time intervals by calculating the reciprocal of the period of a waveform. The Lissajous’ Figure method of measuring
frequency is discussed below.
Phase shifts
5 V/cm
Y1
~
5 V/cm
Timebase
set for
stable
display
Y2
These are most easily measured and demonstrated using a dual-trace oscilloscope
as in the diagram above.
However, to show the phase difference between the voltage across a capacitor (or
inductor) and the current flowing through it, the circuit below would be appropriate. The sensitivity of the Y2 channel is 100 times that of the Y1 channel so that
the voltage across the resistor is negligible compared with that across the
capacitor. Hence Y1, although actually displaying the voltage across the whole
circuit, effectively shows the voltage across the capacitor. (This RC circuit shows
very well the increase in current as the frequency is increased.)
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Phase difference
in an RC circuit
1 nF
~
1 V/div
Y1
5 kHz
150 Ω
Y2
10 mV/div
It is possible to demonstrate these phase shifts using a single beam oscilloscope
with an ‘external trigger’ facility as shown in the circuit below but this is a much
less effective demonstration. The Y input is first connected to C and the oscilloscope is set to give a stable display as shown. Then the Y input is connected to R
and, when adjusted, the display will show a trace starting at a peak.
C
1 nF
~
R
5 kHz
Y
150 Ω
Trigger
selector
to "Ext"
Ext
trig
To measure the velocities of sound and of electromagnetic waves
Many methods of measuring the velocity of sound using an oscilloscope have been
described in text books and the School Science Review. These fall into three groups:
pulse methods; continuous travelling wave methods; standing wave methods.
Pulse methods
These are easiest to understand, being a development of the ‘clap and stopwatch’
method. Nuffield O-level Physics Year IV Expt 93b uses the oscilloscope itself to
generate the pulses from the flyback at the start of the time-base sweep. Some
oscilloscopes do not have a sweep output facility but a step synchronised with
time-base (primarily used for calibration) will do just as well. If the oscilloscope
has no output at all, Mackay1 gives a simple circuit for producing suitable pulses.
Continuous
travelling wave
methods
These compare the phase of the sound wave at the transmitter with that at a
known distance; they are modern versions of Hebb’s method (1904). A dual-trace
oscilloscope allows the phases to be compared without introducing Lissajous’
Figures but these allow more precise location of the in-phase position. Both
techniques are described briefly by Gillespie2.
1
Mackay, R C, Measuring the velocity of sound, School Science Review, June 1974, 55 (193) p785.
2
Gillespie, A, Velocity of sound, School Science Review, June 1973, 56 (197) p780.
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methods
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These require a knowledge of the frequency of the sound. The figure below
illustrates the technique.
Board
λ
Loudspeaker
driven at
known
frequency
Microphone
d
To oscilloscope
input
The separation of the loudspeaker and the board is adjusted for a maximum
variation in signal as the microphone is moved along the axis. A series of minima
are located and a graph is plotted of the distance d against the number of the
minimum (to eliminate errors due to the location of the effective centre of the
microphone). The wavelength is then determined from the slope of the graph. The
velocity is then the product of the wavelength and the frequency.
Nuffield
Advanced
Physics
requirements
Expt 4.7 uses the oscilloscope to measure the velocity of sound in a metal rod but
requires no special oscilloscope features.
This course also uses the oscilloscope in measurements of the speed of electromagnetic waves in free air and in a coaxial cable. These experiments use special
pulse generators and preamplifiers and are described as Expts 4.4 and 8.10. The
oscilloscope may also be used to measure the speed of rotation of the mirror when
measuring the velocity of light (Expt 4.3).
To display Lissajous’ Figures and characteristic curves
The use of Lissajous’ Figures, to set one frequency to an exact multiple of that
generated by an electronically-maintained tuning fork, is no longer necessary, since
signal generators are available with calibrations quite accurate enough for school use.
However, it sometimes remains on school syllabuses.
It is convenient if similar amplifiers are available for both X and Y channels and
this is often achieved by switching one Y amplifier on to the X channel (X-Y
mode). The two channels still have different bandwidths, however, because these
are set by the output stages which drive the deflection plates in the cathode-ray
tube. For this reason, it is usually best to connect the higher frequency to the Y
channel and the lower one to the X channel. The method of calculating the
frequency ratio is given in many standard texts (eg, Hickman, 19901).
It is also possible to use the X input facility to display characteristic curves for a
number of devices or circuits. The top circuit below can be used when a ‘trace
invert’ facility is available or the bottom circuit when it is not. In this case, R
1
Hickman, I, Oscilloscopes How to use them, How they work, Heinemann Newnes but available from RS Components, 1990,
ISBN 0434908088.
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must be chosen so that the voltage across it is small compared with that across
the device and this may be difficult.
Signal
generator
100 Hz
(approx)
Device
Y (inverted)
X
Trace invert facility
Signal
generator
100 Hz
(approx)
Device
Y
R
X
No trace invert facility
12.14.2 To teach ‘the Oscilloscope’ and ‘the CRO’
Probably the best way to teach about cathode-ray tubes and their use in an oscilloscope is to use the Unilab Modular Oscilloscope Kit and build up the system block-byblock in front of the class. However, it is also possible to use a general-purpose
oscilloscope, one technique being to conceal some controls behind cards in order to
concentrate the attention of the class on the section under discussion.
12.14.3 Use with VELA
VELA has an analogue output which gives a voltage between -2.5 V and +2.5 V under
software control. It is most frequently used to provide an analogue representation of
the data held in the memory at the end of the datalogging exercise.
VELA does this by reading each stored value in turn, converting this value to an
analogue voltage and feeding it to the output terminals for a short interval of
time. When all the memory locations have been scanned in this way, VELA goes
through them again. An oscilloscope connected to the output sockets can then
display the values as a stepped curve and may be able to give a steady trace if the
first value happens to be a big one. In case it is not large enough to trigger the
time-base, another output, ‘Synch’, is provided. The program in VELA generates a
+5 V pulse at this output, just before it starts the analogue output, so that a
connection from this socket to the external trigger input on the oscilloscope can be
used to trigger the time-base.
12.14.4 Explanation of terms and features
Display size
This is the size of the visible part of the screen expressed as the diameter if
circular or the diagonal length if rectangular.
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Deflection plates
X
Y
Beam
Final anode
Gun
Screen
Graticule
This is the pattern of lines engraved on the inside surface of the tube face or on a
plastic sheet held in close contact with the screen. The area over which reliable
measurements can be made may be smaller than the graticule. Some graticules
are illuminated by shining light into the edges of the sheet so that the lines glow
brightly on a darker background but this is not common on the cheaper instruments used in schools.
Trace rotation
This control is provided on some instruments to allow the trace to be aligned with
the graticule.
Phosphor
This is the material which glows when struck by the electron beam. Phosphors
glow with different colours and the time for which the glow continues is expressed
by the ‘persistence’.
Phosphors used on school oscilloscopes:
Type
Filter colour
Persistence
P7
Orange
Long
P11
Blue
Fairly long
P31
Blue/green
Fairly short
The P7 phosphor is recommended only where most of the usage requires timebase speeds of less than 20 ms cm-1. These applications are now better covered by
a datalogger with computer read-out.
Double beam or
split beam
oscilloscopes
These use a special cathode-ray tube with two guns or with one gun whose beam
is split. Because these are more expensive and less satisfactory than a dual-trace
oscilloscope, they are now obsolete.
Dual-trace
oscilloscope
This produces two traces from one beam by electronic switching. The beam can be
switched rapidly from one trace to the other and back as the beam crosses the
screen. This is called chopped mode and is useful at low sweep speeds. In
another mode, the beam can be switched to the two signals on alternate sweeps.
This is called alternate mode and is most useful at high sweep speeds. (A
multitrace system works in a similar manner to produce more than two traces.)
The mode is sometimes selected automatically as the sweep speed is changed; in
other cases the user selects whichever mode gives the clearer display. Other
modes often included are add, where the two signals are added, and single where
one trace only is produced. It is often possible to invert one trace which, with the
add function, gives the possibility of subtraction.
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The electronic switches used to produce multitrace operation may be internal or
external to the oscilloscope. An external switch is often called a ‘beam splitter’. A
crude beam splitter can be constructed from modules in the Nuffield A-level
electronics kits but its performance should not be compared with that of the
sophisticated beam splitters.
Input sockets
BNC
UHF
4mm
On school oscilloscopes, these are usually of one of the types shown in the
diagram. Adaptors are available from component suppliers to allow interconnection between systems but it is better to use a screened lead with crocodile
clips at the remote end than an adaptor on the oscilloscope. BNC connectors are
preferred for work at the highest frequencies or with very fast pulses. These are
now almost universal. UHF connectors are very satisfactory for most school work
because the centre socket accepts a 4 mm plug while allowing a screened lead to
be fitted when required. They are not suitable for high frequency work because
they are a non-constant impedance design and will not match the standard
probes. 4 mm connectors are satisfactory for much school work but trouble may be
experienced with small signals, high frequencies and fast pulses. Most 4 mm
socket terminals have a transverse hole which is suitable for use with a 2 mm
plug.
Input impedance This is the load which the input of the oscilloscope presents to the circuit being
measured and is usually quoted as a resistance in parallel with a capacitance (eg,
1 MΩ in parallel with 30 pF). The input impedance should have a large resistance
value and a small capacitance value compared with those in the circuit being
studied.
Probe
This is often used to connect the input to the circuit being studied. Some probes
are passive and provide convenient connectors which reduce waveform distortion
(a ×1 probe) while other passive probes provide an increased input impedance but
a reduction in the signal voltage presented to the oscilloscope; a ×10 probe
provides an output which must be multiplied by 10 to obtain the voltage being
monitored (useful at high frequencies or when testing fast pulses). Active probes
change the impedance which is presented to the circuit being studied without
reducing the signal voltage. They are required in schools only rarely.
Coupling
The vertical deflection system (Y channel) on most oscilloscopes has an associated
switch labelled ‘dc/ac’. When ‘dc’ is selected, there is direct coupling between the
input socket and the deflection plates so that a steady voltage produces a steady
beam deflection; this is the usual condition of use even when examining alternating signals. If the signal consists of a small variation in a large steady level, ‘ac’
can be selected to allow the small variation to be amplified and displayed without
amplifying the steady level; this is ‘ac coupling’. A third position, ‘ground’
(GND), is often provided. This short-circuits the input to the channel (preferably
not the circuit under test) to assist in setting zero levels.
Bandwidth
This is a measure of the frequency response of an oscilloscope and expresses the
range of frequencies over which the amplification or gain of the system is
reasonably constant. As shown below, the bandwidth is the range of frequencies
between the ‘3 dB points’ or between zero frequency and the upper 3 dB point. The
3 dB points of any amplifier are the frequencies at which the gain has fallen to
three decibels less than the value at some reference frequency, ie, the power
available has fallen to half its maximum value.
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Bandwidth
Gain
Bandwidth
Gain
3 dB
dc coupled
3 dB
3 dB
ac coupled
Ref frequency
Frequency
Ref frequency
Frequency
An oscilloscope with a bandwidth from 0 to 5 MHz will clearly be unsuitable for
displaying a 10 MHz signal but it can also distort pulses with a fundamental
frequency much lower than 5 MHz. A pulse with a transition from one level to
another in an infinitely short time (eg, a perfect square wave) requires an infinite
bandwidth. In practice, perfect square waves are displayed as below.
The rise time, t s is related to the bandwidth B Hz by the approximate expression
0.35
t= B
If B = 5 MHz, t = 7 × 10-8 s, ie, 70 ns, which would produce a distinct rounding of a
pulse 1 µs long. For the rounding to be insignificant, t must be about 20 ns, ie, B
must be 17 MHz, requiring a more expensive instrument. The advent of microprocessors has increased the use of fast pulses and hence the need for a high
bandwidth.
Square pulse
10
9
Rounded pulse
1
t
Sensitivity
The sensitivity of an oscilloscope is the ratio of the applied voltage to the vertical
or horizontal deflection it produces. It is usually quoted as a number of V/cm or
mV/cm. Most instruments have a switch which allows the user to select from a
number of different sensitivities but, on some instruments, this quantity may be
continuously variable. The sensitivity control may be referred to as the input
attenuator (because it operates by attenuating it, ie, reducing it, and feeding it
into an amplifier of fixed gain).
Shift controls
These are provided so that the user can move the traces over the screen in order
that features may be more readily compared or aligned with the divisions of the
graticule. If the shift controls are set to extreme positions, the traces can be removed entirely from the screen. It is sometimes difficult for the beginner to
centralise the traces again and a locate or beam finder facility is then very useful: the operation of a push-button reduces the sensitivity so that the traces reappear on the screen. The shift controls can then be used to centralise the traces
before returning to full sensitivity.
Trace inversion
This is useful when comparing two oscillations one of which has become inverted
because of input requirements; see the section above on Lissajous’ figures.
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Time-base
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In most applications of the oscilloscope, the beam is deflected horizontally at a
constant speed. The signal which is applied to the X-deflection system to produce
this is called the time-base.
There are two different types of time-base circuit, one with a free-running
sawtooth generator whose frequency can be set to a particular value and the
other in which the velocity of the sweep is controlled.
Deflection
Sweep
Flyback
Controlled frequency
Time
Deflection
Sweep
Flyback
Rest
Controlled velocity
Time
Synchronisation
In order to obtain a stable display (a steady picture), the beam must trace out the
same path on the screen at regular intervals related to the frequency of the signal
being displayed. With a controlled-frequency time-base, this is achieved by
synchronisation, a fine adjustment of the time-base frequency to make it equal
to (or a multiple of) that of the signal. This makes accurate measurement of time
difference more difficult because it results in the time-base becoming
uncalibrated; it is now much less common.
Triggering
With a controlled-velocity time-base, the sweep begins when the circuit receives a
triggering pulse, the rest period being as long as necessary. The operator sets the
oscilloscope to produce a triggering pulse at a particular signal level and either on
its positive slope or negative slope. If the signal never reaches the level set, the
screen normally remains blank. This is sometimes inconvenient, and many
instruments have a ‘bright line auto’ facility which ensures that a bright line is
automatically displayed in the absence of a triggering signal.
The signal from which the triggering pulse is generated may be the signal being
viewed, if INT (Y1 or Y2) is selected, or it may be a different one. Selection of
LINE generates a trigger pulse from the mains, while EXT selects a signal
connected to a separate input socket.
TV
Many oscilloscopes also have a position marked ‘TV’. A television video signal not
only contains the information which controls the brightness and colour of the
picture, but also contains two sorts of synchronising pulses, one sort for each line
and another for each picture frame. When ‘TV’ is selected, this allows the
triggering pulse to be generated once for each TV frame and is a facility which is
often required in TV servicing.
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Hold-off & delay
The more sophisticated models now available include hold-off and delayed sweep
controls. These enable complicated signals to be examined in detail. The hold-off
control allows the rest period between sweeps to be increased so that more stable
triggering of complex pulse waveforms can be achieved. The delay control allows
a part of the waveform to be expanded to fill the screen, the delay time being the
interval between the trigger pulse and the actual start of the trace. These controls
are not really necessary in school oscilloscopes.
X-Y
In some applications, a time-base is not required: it can be switched off and
another signal used to move the beam in the X-direction. The sensitivity in the Xdirection is often less controllable than that in the Y-direction but many
instruments now have an X-Y facility in which one of the two Y inputs becomes
the X input so that the two channels have identical facilities (although the
bandwidth is usually still different).
Z-modulation
The brightness or intensity of the beam can be set by a panel control to suit the
lighting conditions of the workplace. However, the brightness can be modulated
(or varied) by an electrical signal fed into the Z-modulation input (often on the
back of the instrument). This allows part of a display to be emphasised or
concealed or can add marker points at regular intervals. On some instruments
this feature is provided via an amplifier so that small signals are sufficient; on
others a large amplitude is required to drive the tube directly.
12.14.5 What to buy
Ideal specifications
These specifications are intended to give the minimum performance required. Extra
features and facilities which give rise to further controls and complications in use are
not really wanted. However, the number of oscilloscopes sold to schools is small compared with the number sold to industry and school versions are usually simplified
standard models. It is rarely possible for a manufacturer to meet these ideal specifications exactly, hence the usefulness of CLEAPSS Guide R31, Oscilloscopes for
Schools, which assesses the extent to which the oscilloscopes commercially available
meet them.
All oscilloscopes must meet the electrical safety standards appropriate for the
intended use.
Numbers and types of oscilloscopes required
It is difficult to give rules for the number and types of oscilloscopes which will be
required in schools of different types because it depends on the courses which are
being taught and on the teaching methods used. However, one can give some broad
advice.
The first requirement is for an oscilloscope which meets the demonstration
specification. This should meet the basic needs of the teacher whichever courses are
being taught.
The second priority, where more than six pupils are preparing for A-level physics, is a
second oscilloscope for advanced pupil use. This is similar to the demonstration
oscilloscope but with the specifications less stringent as indicated in Table 12.9.
However, the clarity of the layout of the controls is particularly important as is a
‘trace locate’ facility. It should be a dual-trace instrument as single-trace instruments
for use at this level are a false economy. The numbers should be increased at a rate of
at least one per six pupils.
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Table 12.9
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Demonstration and Advanced pupil oscilloscopes
Features
Demonstration
Advanced pupil
As with the Demonstration
Oscilloscope but:
Display
Flat-faced tube, at least 100 mm diagonal (130 mm desirable) phosphor
and graticule chosen to give a bright, clear trace over the whole timebase range, controls of brilliance and focus, graticule with major
divisions every cm.
Trace
Switch-selected modes: single, dual (chopped), dual (alternate). A A ‘trace locate’ facility is
‘trace locate’ facility is welcome.
particularly welcome.
Y-amplifiers Inputs
BNC sockets with either an adaptor giving 4 mm sockets or a screened
lead terminated with 4 mm plugs or crocodile clips. A ×10 probe should
be available if not supplied.
Input
1 MΩ (parallel capacitance less than 35 pF).
impedance
Bandwidth
DC coupled - dc to 20 MHz.
Only to 10 MHz.
AC coupled - 10 Hz to 20 MHz.
Sensitivity
Switched in 1 - 2 - 5 sequence.
5 mV/cm to 10 V/cm ± 3%.
Shift
control
Must allow an offset of 10 V at a sensitivity of 1 V/cm (ie 20 div).
Linearity
The maximum change in sensitivity across the screen which can be
tolerated is 5%.
Optional inversion of one trace is useful.
Time-base
X-Y facility
Ideally achieved by switching one Y amplifier on to X deflection, since Can be omitted.
this avoids the necessity for extra controls, but can be provided with a
fixed sensitivity and reduced bandwidth. Direct coupling is preferred.
Speeds
Switched in 1 - 2 - 5 sequence from 100 ms/cm to 1 µs/cm. A continuously variable speed control is not required.
Switched multiplier (×5 or ×10) is often useful.
Triggering
modes
Y1 , Y2 or external (internal 50 Hz is not essential) on positive- or
negative-going slopes with bright-line auto which can be disabled to
give a ‘normal’ (pure triggered) mode.
Triggering
sensitivity
Internal - 2 mm screen deflection at 1 kHz.
5 mm screen deflection at 10 kHz.
External - not more than 1 V into 100 kΩ.
Shift
control
Other
features
Must allow the beam to be positioned at any part of the screen and
must allow the whole time-base trace to be examined.
Tube or trace to rotate to align trace with graticule (on rear or side if
convenient).
Time-base ramp output is useful, a synchronise pulse less so.
Not necessary.
Z-modulation input.
Not necessary.
For work to GCSE-level in physics or electronics, oscilloscopes meeting the pupil
specification will be required; details are given in Table 12.10 but, overall, it must be
easy to use, be a reasonable introduction to oscilloscopes and have a low cost. There
should be enough for at least one per four pupils if they are to be used by a class of
pupils working simultaneously.
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Table 12.10
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Pupil oscilloscope
Features
Junior pupil
Display
70 mm diagonal with graticule (5 mm divisions preferred).
Controls of brilliance and focus.
Single beam.
Y-amplifiers Inputs
4 mm sockets, floating with respect to earth and instrument case.
Directly coupled with input impedance of 1 MΩ.
Time-base
Bandwidth
DC to at least 10 kHz, 100 kHz preferred.
Sensitivity
Gain continuously variable or switched in 1 - 2 - 5 sequence. It must be possible to
achieve 100 mV/cm and 10 V/cm with approximate settings marked as mV or V per
division.
Shift control
Must allow an offset of 10 V at gain of 1 V/div (ie, 20 div); more would be an advantage
unless ac coupling is also provided.
Speeds
Off, 100 ms/div, 10, 1, 0.1 ms/div switched (assuming 1 div = 5 mm).
Triggering modes
Automatically triggered from Y input or external source (with bright line auto).
Triggering sensitivity
External input: sensitivity better than 500 mV into 1 MΩ.
Shift control
Must allow the beam to be positioned at any part of the screen.
Other
features
X-amplifier.
Input via 4 mm sockets on rear.
Directly coupled, input impedance 1 MΩ.
Bandwidth: dc to 100 kHz.
Gain: 500 mV/div.
12.15
Pulleys and hoists
Pulleys are used to illustrate the behaviour of machines. They can be obtained from
the main school laboratory suppliers. As well as stocking light plastic or metal pulleys,
which can be assembled to make up simple systems, suppliers also stock hoists, which
since they are designed for lifting heavy loads, are more realistic; these can also be
obtained from car accessory shops such as Halfords.
Pulley block in
line, CLEAPSS
pattern
This is stocked by Philip Harris. Its advantages over most educational pulley
blocks are that it can be used as a one, two or three pulley block and that the cord
is merely passed over the rims of the wheels; it does not have to be threaded.
See also section 12.1 (Beams and rings for lifting).
12.16
Ray sets
A ray set is used to show the path of light through optical systems ranging in
complexity from plane mirrors to telescopes. It does this with narrow beams of light, ie
the ‘rays’, which graze a sheet of paper on the bench or a screen and so indicate the
path of the light. Ray sets usually need partial black-out. Most use a 12 V lamp, often
24 W although some use 36 W. They can be powered by general low-voltage units, the
maximum number of lamps supplied by one of these depending on its rating, usually 4
to 8 A. A unit supplying up to its rated value will often get quite hot but, as it is
protected by a cut-out, this should not cause concern.
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Ray sets should be able to give one ray for some work: eg, for investigating simple
reflection with a plane mirror or refraction by a rectangular glass or plastic block.
They must also give three or more rays for investigating converging and diverging
mirrors and lenses. It is important that the set can, by adjustment, make these rays
divergent or parallel. It is sometimes necessary to adjust the height of the lamp so
that rays skim along paper placed on the bench.
Ray box or
separate
components?
The Nuffield Physics course advocated a ray set with separate components because it was more flexible than a ray box. However, such a set is too complicated
for some pupils; it is necessary to adjust a lamp, a screen with slits, a shade to cut
down stray light etc to produce clear rays before considering the components to be
investigated. Most ray sets available now are much easier to set up, use and clear
away but are still sufficiently versatile.
Bulbs
The 12 V 24 or 36 W bulb used usually has to have a straight filament which is
arranged to be parallel to the slit; this usually means that the filament is parallel
to the axis of the bulb, unlike ripple tank bulbs whose filaments are perpendicular. Such bulbs are quite expensive and it can be worth buying them from an
electrical wholesaler. Care must be taken to purchase the right type of bulb.
m
Lamp housings can become very hot.
Accessories
It is assumed that a basic ray set has a lamp, screens with one slit and three slits,
adequate shielding and perhaps a lens to give parallel rays. To cover the full
range of investigations, the following will be needed:
mirrors: plane and cylindrical (parabolic also useful);
glass or plastic blocks: rectangular and semicircular;
glass or plastic prisms: equilateral and 90-45-45°;
lenses, cylindrical: biconvex, both long and short focal length (eg, 7D and 17D)
and biconcave (-17D);
a screen with many slits (a ‘comb’) is also useful;
holders for all these items.
Colour mixing
12.17
Some ray boxes have a facility for demonstrating the mixing of lights of different
colours. This consists of filter holders to take filters for primary or secondary
colours and so produce three wide beams of different colours which can be made
with plane mirrors to overlap on a small vertical screen. If this facility does not
detract from the main purpose of the ray box nor add substantially to the cost, it
is worth having. Colour mixing can also be investigated using three separate ray
sets, each with a different filter.
Ripple tanks
Ripple tanks are used to teach aspects of wave motion. Most teachers will use them for
pupils to explore basic concepts of waves but with careful technique they can be used
to investigate a very wide range of wave phenomena. Their use in schools was stimulated by the Nuffield Physics Project and many of the ideas here come from Nuffield
publications1.
It is possible to purchase video cassettes of ripple tank investigations and also
computer simulations of wave motion for some computers.
The Nuffield Project advocated ripple tanks with water areas 40 - 45 cm square, intended to be used for pupil investigations with one per group of four pupils. Similar
sized ones are still available but there are also smaller ones, some of which produce
1
Revised Nuffield Physics Teachers’ Guide Year 3 Longman, 1977, ISBN 0582046831, p8.
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reflected images and others which can be used for demonstration on overhead projectors as well as on their own. Nuffield-pattern ripple tanks have a clear 12 V bulb fixed
about 50 cm above the water surface which produces an ‘image’ on the floor below the
tank on a sheet of paper or white-painted hardboard; the image should be viewed
directly, not through the water.
12.17.1 Features, accessories and their use
Legs and
levelling
The legs need to support the tank firmly at about 50 cm above the floor but must
be detachable for ease of storage. There should be a means of levelling the tank,
important for some investigations; one method is to insert window wedges under
the legs until the reflected images of the lamp made by the glass and water
surfaces coincide.
Beaches
By these are meant gentle slopes of the bottom of the tank round the edge, to
reduce reflections. The same effect can be achieved with humps of gauze of metal
or plastic.
Barriers and the
refraction plate
Barriers are metal strips to reflect waves or to produce gaps through which waves
can be diffracted. There are usually three straight barriers supplied: with a
Nuffield-pattern tank, two will be about 15 cm long, one about 2.5 cm, and there
will be a circular barrier, with a chord length of about 22 cm. The height of the
barriers is about 2.5 cm and they have vertical flanges at their ends to keep them
upright.
Supplied to demonstrate refraction is a glass plate, about 30 × 20 cm and 4 mm
thick (plastic plates tend to be too light and inclined to move). Its purpose is to
reduce the depth of the water in an area of the tank; it should be placed on three
small brass washers or something similar to keep it from sticking to the bottom.
Rod and waterdropper
A wooden rod about 15 mm in diameter can be rolled to and fro in the water to
produce straight ripples. Drops from a dropping pipette will produce circular
ripples.
Vibrator bar
This is a bar whose length is slightly less than the width of the tank; on it is
mounted a small electric motor with an off-axis load on its spindle. The bar is
suspended by two rubber bands from a gantry or other supports mounted on the
tank; it is sometimes worth experimenting with different rubber bands. To
produce continuous plane waves, the bar is adjusted just to touch the water but
one or more dippers can be fitted to produce continuous circular waves.
The motor is usually powered by 1.5 to 4.5 V dc; the voltage applied needs to be
adjustable to produce various frequencies of vibration and hence wavelengths. A
special unit can be purchased for supplying both the motor and the lamp but
general-purpose low-voltage supplies are often adequate for the motor if used with
a rheostat. If a special unit is used, care must be taken not to connect the motor to
the lamp supply.
If the off-axis load supplied with the motor is lost, one can be improvised by cutting down the pin of a 13 A plug or a connector from a connector block. Replacement motors from the original supplier tend to be expensive; they are cheaper
from firms who supply small electrical and electronic components for school
science and technology.
Lamp
The lamp must have a small light source and so, usually, a clear 12 V 24 W bulb is
used. It usually has a straight filament, arranged parallel to the vibrator bar so
that it is parallel to plane waves. Usually the filament is perpendicular to the
axis of the bulb; such bulbs should not be confused with those suitable for ray sets.
It should be mounted about 50 cm above the tank and have a shade to reduce
general illumination in the room. If fringes are very faint, eg, after refraction,
their visibility can be improved with some patterns of lamp by rotating it.
© CLEAPSS 1992
Table 12.11
Effect
Pulse of circular
ripples
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Effects which can be demonstrated with a ripple tank
Equipment
Diagram
Water droppers
(bulb pipettes) or
pencils
Note
A drop of water or touching the water with a pencil or finger
generates a pulse of two or three circular waves. Does water
move along with the pulse?
X
It is important to show that, if two pulses are generated simultaneously, they will pass through each other unaffected. It is
probably best to show this later.
Pulse of straight
ripples
Wooden rod
The rod is rolled forwards and backwards. Alternatively, a
12 inch ruler can be dipped into the water or put in and then
oscillated forwards and backwards.
Reflection at
barriers
Water dropper etc
for circular pulses
The reflection of both circular and straight pulses can be investigated. With circular pulses, pupils can be asked to mark with
a small object where the reflected pulse appears to come from.
The position can be adjusted to look right and the distances
compared. The angle that straight ripples make at the barrier
should be varied (avoid 45°).
Wooden rod for
straight pulses
Barriers to act as
reflectors
Continuous
waves
Vibrator bar and
adjustable dc
source
Hand stroboscope
X
Better focusing of plane waves can be obtained if a parabolic
reflector is made by putting copper wire in rubber pressure
tubing and bending it into a parabola, using a template.
The height of the bar must be adjusted so that it just touches
the water. The frequency of the motor should be varied by
changing the voltage applied and it will be observed that the
higher the frequency of vibration, the shorter the distance between the waves, ie the wavelength.
If the waves are ‘frozen’ with a hand stroboscope, frequency
and wavelength can be measured and fλ = constant shown.
Interference
Vibrator bar fitted
with two dippers
In the diagram, lines connecting maxima or minima are shown,
not wavefronts. The pattern observed can be changed by
varying the frequency of vibration of the bar and by varying the
distance apart of the dippers on the bar. Changes in pattern
are easiest to see if the frequency is not too high.
With several vibrators, it is possible to show the effect of a
diffraction grating.
Diffraction
Vibrator bar
Hand stroboscope
The pattern can be changed by varying the frequency of
vibrations and the width of the gap. Again, vibrator speeds
should be kept low to start with. At higher speeds, the
stroboscope may aid visibility.
Other arrangements can be tried: double slits, giving Young’s
fringes, and an obstacle, eg, the short barrier, instead of a gap.
Refraction
Vibrator bar
Glass plate resting
under water on
three washers
Hand stroboscope
If the plate is placed directly on the bottom of the tank, it is
inclined to stick there. The tank must be level and the plate just
covered with water. Refracted waves should be visible at the
front of the plate but the shallow water, necessary to produce
the change in velocity, will damp them rapidly.
Adjustments may be necessary to make the waves visible. The
height of the vibrator and the direction of the filament of the
bulb are both critical. The frequency should be kept low.
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Stroboscope
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A simple hand stroboscope is required to ‘freeze’ waves. It consists of a hardboard
disk, slotted round the edge and pivoted at its centre on a handle. Normally, it is
rotated by being held in the left hand and turned with the first finger of the right
hand in a hole. The waves are viewed through the edge of the disk.
Some patterns of ripple tanks, eg, those which work on overhead projectors, have
motor-driven disk strobes as accessories. These are mounted just below the lens of
the OHP.
12.17.2 Use of ripple tanks
The Nuffield project advocated allowing pupils to investigate for some periods with
little direction but not all teachers will agree with this policy or find that they have
time for it. Considerable technique is needed to observe some phenomena clearly.
Whatever the general view on direction of pupils for this work, it is sensible if setting
up and taking down ripple tanks is done quickly and efficiently following a drill. Some
spills must be expected and sponges, buckets etc provided.
Setting up
Teachers may wish to inspect that legs are secure before water is added. It is
convenient to have containers, eg, cut-down squash bottles with a mark at the
level it should be filled to; around a litre will give the required depth in the tank,
about 5 mm. The tanks and the water must be clean. A little detergent (eg,
washing-up liquid) added to the water will help reduce stray reflections and
improve what is seen.
Taking down
Thought must be given to emptying. If sinks are too small to allow easy pouring, it
may be necessary to use one or two containers such as plant troughs. These can be
emptied subsequently or have holes made in them to empty directly into sinks.
A reminder of some of the effects which can be illustrated with brief notes on the
equipment needed etc is given in Table 12.11.
Work without the Much useful work can be done without the vibrator bar, with a group of a few
ripples being generated manually.
vibrator bar
Stray ripples
In some demonstrations, what is seen can be improved by using extra barriers to
prevent ripples passing round the edges of the main obstacles.
Refraction
It is difficult to make this demonstration clear as the depth of water is very
critical. While water (gravity) waves do get slower as the depth is reduced, surface
tension becomes the main factor at small depths and the speed increases.
12.18
Stretched wires etc
Metal wires, springs and nylon filaments are stretched in several investigations: in
sound, the sonometer; in properties of materials, Young’s modulus measurements and
tensile testing; properties of springs.
wires of certain metals (eg, steel and Eureka but not copper), filaments of
m Stretched
certain plastics (eg, nylon) and springs can cause cuts if they break or become
detached while under tension. The eyes in particular are at risk. See section 3.2 (Eye
protection).
Normal laboratory eye protection is advised. Faces should be kept well away from
stretched wires etc but, if this is impossible or if the wire etc is intended or expected to
break, a safety screen should be used. Suspended masses above 2 kg should be
arranged to be as near the floor as possible and over a cardboard box containing
padding etc.
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Stroboscopes
Three types of stroboscope are used in school and college laboratories. A hand
stroboscope consists of a hardboard or plastic disk, slotted round the edge and
pivoted at its centre on a handle. Normally, it is rotated by being held in the left hand
and turned with the first finger of the right hand in a hole. The waves are viewed
through the edge of the disk. A motor driven stroboscope has a similar disk
mounted on a motor whose speed of rotation can be adjusted. A xenon stroboscope
emits flashes of light from a small xenon tube; it is mains powered and its frequency
can be controlled by a switch and a graduated knob.
All three stroboscopes are used for ‘freezing’ periodic motion, eg, of a vibrating string,
or fixing the position of a continuously-moving object at intervals: the first two are
used in conjunction with a ripple tank (see 12.17, Ripple tanks ) and all can be used
for dynamics as in section 12.4.2 (Timing methods).
flashers operating at frequencies around 7 Hz to 15 Hz have been known to
m Xenon
cause epileptic fits. This range of frequencies should be avoided and special care
should be taken if any pupil is known to be an epileptic.
12.20
Vacuum
[Information previously in this section is now in section 10.6, Pumps.]
12.21
Wave machines
Wave machines are used to demonstrate the motion of transverse and longitudinal
waves, both progressive (travelling) waves and standing waves. Wave motion is not
easy to visualise and so such machines are useful. However, these motions can be
represented by simpler means, for example, by a Mexican wave along a row of pupils;
also by computer simulations. These should be considered before a bulky and expensive wave machine is purchased. See Table 12.14.
Wave machines are in two categories, those which are made up of coupled oscillators
and those in which the motion of a spiral is projected mechanically or optically on to a
plane. The former usually has the advantage that alterations can be made to the
system to change the velocity of the progressive wave; reflection can also be shown
and hence the formation of a standing wave by the addition of two progressive waves.
For details of the demonstrations listed in Table 12.14, see Nuffield Advanced Physics,
Teachers’ Guide Waves and Oscillations1. The investigations using trolleys are for
sixth form work.
1
Revised Nuffield Advanced Science, Physics, Teachers’ Guide 1, Longman, 1985, ISBN 058235417X, Unit D.
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Ways of demonstrating wave motion
Apparatus
Types
of
wave
Note
Arrangement
Long spring
Trans The spring is laid on a
smooth floor. The end can
3 m long, 2 cm in
be free or clamped to a
diameter.
bench leg when the phase
Shows transverse waves
of the reflected pulse will be
better than a slinky.
different.
Slinky
The longer model is
better; it is about 11 cm
long when compressed.
Trans As for the long spring but
not so clear.
Long To show longitudinal pulses,
the slinky should be
stretched on a smooth
surface and then a couple of
turns should be pulled back
and released. Or it can be
kept suspended from a strip
of wood by threads.
Transverse wave kit
Trans The cords are stretched
tight and waves generated
60 - 100 wooden rods
by moving a rod up and
about 30 cm long, 8 mm
down.
in diameter, threaded on
two cords between two
handles. There are
spacers between the
rods.
Top view
To
To
handle
handle
Side view
Dynamics trolleys
Trans This is best done on the
floor.
At least 8 are needed.
Also, for each trolley:
For transverse, the springs
transverse waves, Long are attached with pegs or
large nails. The trolleys are
2 springs (6 mm
pulled apart until the springs
diameter, 15 cm
are in tension and then
long) .
individual ones adjusted.
longitudinal waves,
For longitudinal waves, the
2 or 3 open springs
trolleys must be fitted with
(15 mm diameter,
holders to take the ends of
5 cm long) and
the springs.
holders.
The trolleys can be loaded
to alter the frequency.
Trans = transverse
Long = longitudinal
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Magnets
[The information in this section was previously in section 10.1. It has not been revised, other than
to change section numbers. Pending a complete revision of Chapter 12, it is being temporarily held
in this new section.]
12.22.1 Precautions
Magnets can harm some wrist-watches, magnetic tapes for recorders and computer
discs.
We have heard of children trying to obtain magnets from the focusing system around
old TV tubes. These tubes implode when broken and so there is a potential danger
from flying glass. The practice should be discouraged unless carried out under
supervision.
12.22.2 Magnets in science
Secondary schools will require magnets to consolidate the study of their properties
begun in primary schools and to consider their application in electromagnetic devices.
Types of magnet
We recommend that, for their main work on the properties of magnets, schools use
alloy (Alnico etc) magnets. These represent the best compromise between the different
properties. Some weaker, easily de-magnetised and re-magnetised, steel bar magnets
may also be required.
Bar magnets have the most useful shape but it is worth having at least one ‘horseshoe’
magnet.
It is interesting to have a collection of some of the many different types of magnet now
available. A ring magnet floats above another if threaded on a suitable rod or tube. It
is also worth having specimens of tape or sheet magnetic ‘rubber’. The arrangement of
the poles on various types is as shown on the next page.
The most suitable arrangement for producing strong fields for electromagnetic work is
a pair of ferrite (eg Magnadur) strips fitted to a mild steel U-section yoke. This idea
originated in the Westminster Electromagnetic Kit.
Field lines demonstration
Iron filings can be stuck to a card using some hair lacquer or clear lacquer from
electronics suppliers such as RS Components.
Coarse iron filings give a clearer picture than fine ones. The filings can be sprinkled
from a pepper pot with large holes or a 35 mm film container with holes made in its
lid with a hot nail held in pliers. Filings which cannot be pulled off a magnet can be
removed with a piece of plasticine or Blu-tack.
Iron filings are messy and teachers may prefer to keep them in a sealed transparent
‘box’. The box is tapped until filings are spread evenly over the bottom, a magnet
brought up underneath and the box tapped again. Two plastic Petri dishes taped
together are ideal. Experiment with different quantities of iron filings.
N
S
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The arrangements of poles in various magnets
a. Steel or alloy types
Horseshoe magnet
U-type with keeper
Cylindrical bar
Button type
b. Ferrite types
Ferrite bar
Plastic strip
Ferrite ring
Plastic sheet (poles are on
one side of the sheet)
Plastic ‘string’
N
S
N
S
S
N
12.22.3 Care
Magnets can easily become demagnetised or be broken if not handled with care.
The strength of a magnet does not just leak away. However, ‘permanent’ magnets do
lose their magnetisation when they are knocked or when other magnets are brought
near them, particularly if in such a way as to produce repulsion. A strong magnet can
re-magnetise a weak magnet the other way round if brought up close.
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Horseshoe magnets can be stored in pairs but should be separated as shown. Bar
magnets can be stored in pairs as shown below but this is not as satisfactory as with
keepers. If magnets are not kept with keepers or in pairs they should be stored well
apart. A partitioned tray helps to achieve this and aids the check to see that magnets
are returned at the end of the lesson.
N
S
S
N
Ferrite magnets, which are very brittle, can be protected by binding with Sellotape.
Bits of ferrite magnet can be used to stick pictures on to steel cupboards but soon
‘disappear’.
Magnet storage tray
Stored with keepers in place
Wooden separator
Stored with unlike poles adjacent
(less satisfactory)
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Pupil’s explorations of magnets are bound to weaken them and teachers must expect
to have to have them re-magnetised or to replace them. However, after a first
exploration it is worth encouraging them to be gentle with them.
For re-magnetisation of bar magnets, it is necessary to use a solenoid through which a
large electric current is passed (it need pass only for an instant, however). ‘Stroking’
produces only weak magnets.
Horseshoe magnets present more of a problem. These must be placed across the poles
of an electromagnet and as large a current as the electromagnet will stand passed.
Magnets of steel or alloy are usually sold with steel ‘keepers’. If stored with keepers
across their poles, magnets withstand knocks and other magnets being brought close
very much better. Keepers should be placed gently onto the magnets and pulled off
straight, not slid.
12.22.4 Re-magnetising
Solenoids
A solenoid is a long coil which can be used to re-magnetise magnets which have
become weak. They can be bought or made.
Bought
A bought solenoid is quite expensive, particularly if it has its own built-in power
supply. However, a teachers’ centre could usefully buy one to re-magnetise the
magnets of the schools it serves and so we give some information about them.
D-i-y
It is possible to make a solenoid and, with some workshop skill, it can be a good
one: the tube in the middle could be a piece of plastic conduit and the ‘cheeks’ at
either end to stop the turns coming off made from plywood or hardboard.
A simple solenoid made by winding about three layers of 5 A single insulated
cable on a cardboard tube can be quite effective.
Section through
a cardboard tube
solenoid
Wire
Magnet
Cardboard tube
Tape
It can be difficult to arrange for a large enough current through the solenoid to
magnetise alloys such as Alnico.
One method we have used successfully is to employ a healthy car battery. Put a
magnet inside the solenoid, held one end of the cable against one terminal very
firmly and touched the other end on the other terminal firmly, for a few seconds.
The magnet jumps inside the coil, a satisfactory sign that the solenoid was doing
its job.
It is not sensible to re-magnetise too many magnets at once in this way as it might
flatten the battery and overheat the coil. It is worth having two people, one to hold
one end of the wire firmly on one battery terminal, the other to tough the other
end on the other terminal. A car battery gives only 12 V and will not give any
shock on its own, but, surprisingly, disconnecting a solenoid with a magnet in it
can give a small shock. This is not dangerous but might startle you. Having two
people prevents this.
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It is not worth trying to predict which end of the magnet will become the N pole,
which the S pole. If it bothers you, get it right by trial and error.
De-magnetising
A solenoid may be used for de-magnetising if an ac current is passed through it
and the magnet taken out with the current flowing.
12.22.5 Background information
Lodestone
This is a lump of a rock called Magnetite, which can be magnetised. It is an iron
ore, Fe3O4. Lumps of Magnetite if suspended or floated will set north and south,
acting as compasses. In fact, lode (= way or journey) stones were the first
compasses.
Substances
attracted by
magnets
All the everyday substances attracted by magnets are metals but most metals are
not attracted by magnets. Metals are frequently given surface treatments such as
plating which can make them difficult to identify.
Common metals attracted
IRON (Usually met in everyday objects in the form of steel, iron alloyed with
small quantities of other substances. Examples: nails; needles; pins (usually
plated); steel cans (plated with tin); blades of scissors and knives; razor blades.)
NICKEL (Found in laboratories in crucibles and spatulas.)
COBALT (Met only as an ingredient of alloys.)
Common metals not attracted
COPPER (Small diameter pipes; 1, 2p coins which are alloy but mostly copper.)
CUPRO-NICKEL (5, 10, 50p coins. Non-magnetic in spite of the nickel as they are
75% copper.)
BRASS (An alloy of copper mainly with zinc. Some knobs; screws; keys.)
ZINC and ALLOYS (Small toys such as model cars.)
LEAD (Old pipes; fishing weights.)
ALUMINIUM (Saucepans; cooking foil; milk bottle tops.)
GOLD, SILVER
STAINLESS STEEL (Some varieties are magnetic.)
Common
magnetic
materials
Name
Composition
Magnetic
strength
Ability to keep
magnetisation
Resistance to
breakage
Steels
6% chromium
15% cobalt
Alloys
iron with 6% chromium
iron with 15% cobalt
9
99
9
99
999
999
Alni, Alnico,
Alcomax, Ticonal,
Fecal
Ferrites
iron with substantial quantities
of aluminium, cobalt, copper,
nickel, titanium etc
999
99
99
Magnadur, Feroba
chemical compounds of barium
or strontium with iron and
oxygen
99
999
9
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Fecal is less magnetically strong than the other alloys but cheaper. Ferrites are
not metals and are bonded to form materials like china which are similarly brittle.
Ferrite magnets are sometimes called ceramic magnets.
Ferrites can also be bonded with plastic to form flexible ‘rubber’ sheet, tape and
string which can be cut with scissors. In this form, ferrites do not form such strong
magnets.
‘Iron and steel’
As an older text-book would have it: ‘Steel (a harder form of iron) is used for
permanent magnets in preference to soft iron since, although more difficult to
magnetise, it retains its magnetism very much more tenaciously under rough
treatment’. The truth is not so simple.
The point, of course, is that there are many steels, including some specially
developed for magnets. The aim in developing a suitable material for a permanent
magnet is to make magnets which are:
a)
as magnetically strong as possible;
b)
as permanent as possible, ie able to keep their magnetisation, despite being
knocked and having other magnets brought near.
Unfortunately, achieving these qualities usually results in a magnet which is
rather brittle, liable to break when dropped.
Just as any old steel is not suitable for making magnets, so any old soft iron is not
suitable for transformer cores and other situations where ready magnetisation
and demagnetisation is needed. Again, special materials have been developed.