A RECORDING INTERFEROMETER
by
Jacques R.
Maroni
Submitted in Partial Fulfillment of
the Requirements for
the Degree of
BACHELOR OF SCIENCE IN MECHANICAL ENGINEERING
from the
Massachusetts Institute of Technology
1943
Signature of Author.
Department of Mechanical Engineering..............
Date..
. . . ..
..
.
........
ss* .. ..
Professor in Charge of Thesis.
..
...
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.
A C K N O W L E D G E M E N T
The author gratefully acknowledges the
aid of Dr. W. M. Murray in suggesting the
subject, and in offering helpful comments
on the work.
TABLE
OF
CONTENTS
Page
Section I
Introduction
Section II
The Light Source
4
Section III
The Interferometer
10
Section IV
The Light-Sensitive Device
18
Section V
Amplification and Simple Relay Circuits 32
Section VI
Automatic Recording of the
Photoelectric Current
37
Section VII
Conclusion
41
Section VIII
Appendix
46
Section IX
Bibliography
57
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1.
S E C T IO N
INTRODUCTION
L
I
INTRODUCTION
The accurate measurement of strains is an important
part of the tension test. In this respect, the Huggenberger tensometer is a valuable strain measuring instrument
because it is sensitive, rugged, relatively inexpensive
and easy to handle. In 1934, Mr. Vose1 made an extensive
study of the characteristics of the Huggenberger tensometer, and found that, due to wear of the instrument parts,
a calibration was desirable from time to time, for best
accuracy. In that connection, he devised a special interferometer which makes possible a direct comparison of the
tensometer reading against the actual displacement as
measured from the number of interference bands going by
a reference point.
The purpose of this thesis is to determine whether
it is possible to make the use of the interferometer more
reliable and less tedious by providing an automatic recording in place of the eye-straining visual observation.
Such a study has been very much hampered by the fact that
most of the equipment needed for actual tests was unavai-
lable due to the present war emergency. It will therefore
have to be a preliminary survey of the problem, in which
we will only attempt to profit from previous investiga1: A.S.T.M. Proceedings. 1934
3.
tions made on the different pieces of equipment needed,
in such a way as to open the path for future studies.
A definite solution will no doubt have to wait until such
a time that a complete test may be set up.
The equipment needed may be listed under five headings, each of which will have to be considered.
(1) the light source.
(2)
the interferometer.
(3) the light-sensitive device.
(4) the an.plifier.
(5) the recording device.
Numerous studies have been made on these different
items, and we propose in the following pages to indicate
what considerations must be taken into account to direct
and coordinate the necessary selections of equipment.
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4.
S E C T I 0 N
II
THE LIGHT SOURCE
k
5.
THE LIGHT SOURCE
In making the choice of a proper light source, the
requirements are that it be simply and conveniently operated while at the same time giving out a highly intense,
constant and monochromatic radiation. We can easily understand the reason for these requirements: the radiation
must be monochromatic so that the interference pattern
will be black and white, and not colored as it would be
if white light were used. It must be intense , in order
to increase the energy which will be distributed along
the different interference fringes, thus making easier
the detection by the light-sensitive device. Finally,
it must be constant because the detection cannot be expected to be reliable if the light source intensity varies
from time to time.
At this point, it is important to point out that,
in most cases, the light-sensitive device does not respond equally well to radiations of different wavelengths.
It is apparent then, that we will want to use a wavelength of such a magnitude as to correspond to a range
of high sensitivity for the light detecting device. We
see therefore, that the selection of a light source is
rather closely connected with the choice of a light-sen-
4f
6.
sitive device. This means that we will have to make a
first selection which we may have to revise later from
the light detection point of view.
There are several types of light sources which will
satisfy our general requirements:
One solution would be to use a high intensity incandescent lamp with the proper filter arrangement. This
provides obviously a simple and convenient answer but it
has the great disadvantage of requiring filtering to a
point where most of the energy emitted is absorbed.
Another solution is to use the flame of a burning
gas. Alcohol, sodium salts, and acetylene have been used
in the past. This method affords a remarkably constant
and reliable light source, but there again the intensity
is quite low.
A third and better method consists in using as a
light source, an electrically lighted gaseous substance.
This can be done by either one of two ways:
a) gaseous discharges in rarefied atmosphere.
b) electric arcs.
The gaseous discharge method is extremely valuable
in many applications. Lord Rayleigh, Michelson, Fabry,
used it in spectroscopic studies to determine the width
of spectral lines and the maximum order of interference
that could be observed with different vapors. They found
that narrow lines could be obtained only with vapors of
high atomic weight under low pressures and temperatures.
Mercury was used successfully although the best line
(X = 5461 A ) is surrounded by a few " satellites ".
Cadmium gives a very fine red line (=
6438.4696 A )
which is used as a reference in spectroscopic work. The
use of cadmium tubes , however, is not very convenient,
as they have a short life and do not always radiate as
intensely as is desirable. Krypton tubes immersed in liquid air have produced the finest lines ever observed.
Neon tubes give very intense lines mostly in the orange
and red region. This method affords the best light sources for accurate experimental work. However, it requires
a rather high voltage and it is not especially desirable
when extremely fine lines are not necessary.
The electric arc method seems to be more desirable
in our case. Carbon or tungsten arcs are used in several
applications and they provide very intense sources of
light. However they usually require large control boxes.
Furthermore the intense heat generated may injure the
reflector and the required filters. Mercury arcs of the
Cooper-Hewitt type have many distinct advantages. They
provide very bright spectral lines that are so far apart
8.
that they can be easily isolated with filters. They are
produced commercially and can be operated conveniently.
They have proved to be an invaluable tool in innumerable
optical experiments. They send out a light which is characteristic of mercury vapor: in the visible spectrum we
find violet radiations convenient in photographic work,
green radiations desirable for visual observation, and
yellow radiations. There is a deficiency in red radiations. Many styles of mercury vapor lamps have been devised. The tube is usually made of fused quartz and we
may note in passing that all fingerprints on the tube
must be removed before the arc is started. If this is
not done, the salts contained in the grease on the fingers will unite with the quartz when the latter is heated
and cracks in the tube may result. The brightness of the
are stream may be anything from 1 to as high as 1000
candles/cm 2 . The lamp for best results is enclosed in a
lamp house in which is mounted a reflector, so that most
of the light is reflected back on the lamp house window.
The window may or may not be fitted with adjustable shutters, the purpose of which is to regulate the size of the
light beam. In addition, it will be desirable to use a
collimating lens to obtain a parallel beam.
We have up to now put the emphasis on the fact that
9.
the light source should be highly intense. There is, however, a definite limit as to the desirable light intensity; obviously a highly intense light source means that
a great amount of heat is generated and must be radiated
away. This consideration involves a heat transfer problem
which determines a practical light intensity limit.
The recommendations which have been made thus far
concerning the selection of a light source, have been
I
(
motivated in part by our preoccupation to be on the safe
side. It may well be that tests will show the relative
importance of the several factors to be not what we expected. But we know that a perfect black and white interference pattern requires a perfectly monochromatic light
source, and that satisfactory light detection and recording demands a highly constant and intense light source.
These ideal qualities should guide the choice of the light
source. Their relative importance can be determined only
in specific cases through actual tests.
T
10.
I
rI
S E C T I 0 N
III
THE INTERFEROMETER
11.
THE INTERFEROMETER
We can divide the methods of obtaining interference
effects into two broad classes:
(1) Methods which require a point or line source and which produce interference effects by a division
of wavefront.
(2) Methods in which the beam is divided by
partial reflection into two or more beams, and which produce interference effects by a division of amplitude.
Examples in the first class are the Fresnel biprism,
Lloyd's mirror, Billet split lens, and the Rayleigh interferometer. Examples in the second class are the interferometer systems of Fizeau, Jamin, Mach, Michelson, Fabry Perot and Lummer Gehrcke.
The systems in the second
class offer the distinct advantage that extended light
sourcesmay be used, so that in general the effects are
much brighter. In the present discussion, only the systeis of the second class are of interest, due to the fact
that the brightness of the fringe pattern is of paramount
importance.
On the original interferometer devised by Mr. Vose
to calibrate Huggenberger tensometers, the optical system is similar to the one of the Fizeau interferometer.
12.
It consists essentially of two thick glass disks. The
front surface of the first disk is inclined at a slight
angle so that the reflected light does not enter the eye
of the observer and the back surface of the second disk
is either inclined or blackened. Under these conditions
the interference effects are produced by the inner surfaces
of the first and second disks. If these surfaces are exactly parallel and are separated by an integral number of
wavelengths, the point at the foot of the perpendicular
dropped from the point of observation will be dark. This
point is surrounded by alternate bright and dark rings.
As the distance separating the two inner surfaces is increased, the spot at the center becomes alternately bright
and dark. If the two surfaces are not accurately plane,
the rings will have irregular shapes similar to the con-
tour lines of a topographic map.
In practice, this system is not satisfactory for
our purpose. For one thing, the two plane surfaces must
be adjusted continuously for parallelism; for another,
the interference fringes are not sufficiently wide to
make it easy for the light-sensitive device to single
them out. It is important to keep in mind the fact that
the optical system of the interferometer must be such as
to satisfy the requirements of the light-sensitive device.
13.
These are, in general terms, that the interference bands
be of such a width and such intensity as to make possible
the detection of one band at a time. In addition, it would
be advantageous to use a system which does not need to be
adjusted continuously.
If, for example, we used a system consisting of an
optical flat and of a spherical lens, we would obtain a
pattern of Newton rings not very different from the pattern obtained with the Fizeau method when the two inner
surfaces are exactly parallel. There would, however,
no need for frequent adjustment.
Furthermore,
be
we could
make the diameter of the central spot whatever we want,
by choosing properly the radius of curvature of the spherical lens. The longer the radius of curvature of the lens,
the larger the diameter of the central spot. It seems therefore that such an arrangement would be quite satisfactory.
Assuming now that we have decided to use such a system, we can try to determine whether it is best for our
purpose to observe it by transmission or reflection, and
whether partial silvering of the surfaces can improve the
fringe pattern from the intensity point of view. In that
connection, the first property which must be mentionned
is that the patterns observed by reflected and transmit-
14.
ted light are always complementary, the maxima of one
system corresponding to the minima of the other. This
property can be easily verified by placing light sources
of equal brightness in front and behind the apparatus.
The pattern observed by transmission is superimposed
on the beam of light going through the optical system in
such a way, that the dark rings do not appear dark but
merely less bright than the bright rings. The reflected
pattern presents a stronger contrast between bright and
dark rings, but the light intensity is not as high as in
the case of transmission, largely due to the low coefficient of reflection of unsilvered glass. It seems that
it is preferable to observe the pattern by reflection but
it may be that no convenient light-sensitive device is
sensitive enough to detect such a low intensity. If that
is the case, we may have to observe the pattern by transmission.
We may also silver partially the surfaces of the
optical system of the interferometer. The silvering of
the surfaces has, however, the disadvantage of making the
interference phenomenon more complicated and rather unpredictable, due to a series of inter-reflections. As a
general rule, it seems that partial silvering of the surfaces causes the interference bands to appear brighter
15.
but narrower. Such a modification of the pattern could
hardly be considered as a great improvement from the point
of view of detection. It may be proper to note that in
the case of silvered surfaces, some light is absorbed in
the silver film. which means that the reflected and transmitted patterns are no longer complementary.
One disadvantage of the simple system consisting of
a spherical lens and an optical flat is, that, when the
angle of incidence is increased, there is a
considerable
loss of light by reflection at the first glass surface.
A large angle of incidence is decidedly detrimental to
the brightness of the rings. This difficulty may be avoided by using a system consisting of a prism and a lens.
One face of the prism is placed against the curved surface of the lens, so that light falling nearly perpendiculary on one of the other faces will enter the prism in
large quantity. This arrangement has the additional advantage of making large the angle of refraction into the
air film between the prism and the lens, which increases
the radius of the rings as it depends not only on the radius of curvature of the lens, and on the wavelength used,
but also on the secant of the angle of refraction into
the air film. Viewed through the third face of the prism
the rings obtained in this manner are both bright and
large. The same result may be obtained with a glass plate
16.
and a prism having one face polished into a spherical
form of small curvature.
In any of these systems where prisms are used, we
have to take into account the effect of dispersion. It
is interesting to note that, especially in the neighborhood of total reflection, dispersion has an achromatising
effect. In other words, the bright rings, obtained with
white light are nearly white instead of being highly colored. We see then, that it may be possible to use a
prism-lens system with an intense source of white light,
and with no filters required. It may be that such a system
would actually be more satisfactory than a plate-lens system.
Summing up this discussion, we can say that there
seems to be a definite advantage in the use of a platelens system in place of a two plate system. The plate-lens
interference pattern will show a greater contrast between
dark and bright rings if viewed by reflection. If however,
the light intensity is too low to be easily detected by
a convenient light-sensitive device, that situation may
possibly be improved by proper partial silvering of the
surfaces. If silvering turns out to be impractical it is
possible to view the pattern by transmission, in which
case the actual recording of interference bands is likely
17.
to be satisfactory only if the light-sensitive device
has a linear characteristic over the range of illumination. Finally, it is possible that the best results will
be obtained with a prism-lens system which can give bright
and large achromatic rings even when the angle of inci-
dence is large, and when the light is not monochromatic.
r
18.
S E C T I 0 N
IV
THE LIGHT-SENSITIVE
DEVICE
I
I
19.
THE LIGHT-SENSITIVE DEVICE
In the past, a number of more or less satisfactory
instruments have been used to measure radiant energy.
There are non-selective radiation meters; e.g., thermopile, thermocouple, radiometer and bolometer, wherein
radiant energy is absorbed by a blackened receiver and
converted into heat. Also, there are selective light-sensitive instruments.
The automatic recording of the number of times a
certain spot in the interferometer system changes from
dark to bright illumination requires the use of one such
device. It is apparent however, that selective light-sensitive cells are better suited to our purpose, not only
because of their selective capacity, but also because of
the many qualities which have been incorporated into them
through many years of development.
A light-sensitive cell is essentially a device by
means of which light energy can be used to control electrical energy. There are two broad classes of light-sensitive cells: the first class is said to be photoconductive. It includes the cells which merely change their
electrical resistance when illuminated. The outstanding
example is the selenium cell. The second class is said
20.
to be photoemissive. It includes the cells which exhibit
the photoelectric effect. A photoelectric cell is essentially a light-sensitive material which, placed inside
an evacuated or gas-filled bulb, emits electrons when exposed to radiant energy of short wavelengths, thus causing a current to flow to a positively charged collecting
electrode in the bulb.
WIe will immediately eliminate the selenium cells.
They have a high sensitivity which in certain instances
may make current amplification unnecessary, but that quality is more than counter-balanced by several grave disadvantages. These are:
(1) They have an appreciable and sometimes unpredictable lag in response.
(2) Their resistance varies greatly with temperature.
(3) They are vulnerable to moisture and shock
and the constancy of their behavior is uncertain.
(4) They do not have a straight line characteristic.
We may now confine our attention to cells of the
1: The increase in current due to the action of the light
is very nearly proportional to the square root of the illumination, in the case of selenium cells. This may not
necessarily be a disadvantage, however.
21.
photoemissive class. This class is essentially made up
of the so called photovoltaic and photoelectric cells.
A satisfactory cell in this class should have the following desirable characteristics:
(1) Constancy and speed in response.
(2) A definite and constant relationship between
intensity of illumination and current given off.
(3) A high spectral sensitivity well fitted to
the light source used.
(4) Low " dark current ".
Photoelectric Cells.
The photoelectric cells have been used extensively
in many applications, and we will consider them first.
However, as there is a great number of photoelectric cells
available, it may be in order to see what the factors
which influence their characteristics, are.
The first factor we will mention, is the one which
determines the ratio of the primary photoelectric current
to the incident light: it is the material of the cathode.
When the cell is to be used with visible light, our choice
in this matter is closely limited; for there are only
seven metals known that are sensitive at all to such light.
They are sodium, potassium, rubidium, caesium, lithium,
strontium and barium. All these metals oxidize rapidly
22.
in air and must be prepared in a sealed vessel. Only the
first four can be distilled in glass; the difficulty of
preparing the three others is such that they have almost
never been used.
The second factor determines the proportion of the
incident light which falls on the cathode: it is the area
of the cathode and of the window. This factor is not too
important as long as the window and cathode are large
enough to admit the whole beam of light. It is known that
it makes practically no difference how the light is distributed over the cathode of a photoelectric cell. The
cathode material is usually deposited on a spherical or
on a plane surface. The shape of the cathode surface has
apparently a definite influence on the cell characteristics.
The third factor has to do with the conditions inside the photoelectric cell. The cell may be either pumped out and sealed; or it may be filled with some inert
gas. The presence of the inert gas increases appreciably
the sensitivity of the cellas the primary photoelectric
current is magnified by the process of ionization by collision. The magnification can be increased or decreased
1:This is not true in the case of a selenium cell.
23.
within certain limits by varying the pressure inside the
bulb, and the applied electric field.
There are many other factors of lesser importance,
some of which are not yet completely understood and are
outside the scope of this discussion. The factors mentionned above will be essential in guiding the choice of
a cell.
Vacuum versus Gas-filled Photoelectric Cells.
The vacuum cell is theoretically the simplest form
of photocell.
With a given amount of luminous energy in-
cident on the cathode,
the vacuum cell will show a gradu-
ally increasing current with increasing applied voltage.
As the voltage is further increased however, the current
reaches soon a limiting value known as saturation current.
The gas-filled cell has a similar current-voltage
relationship for low voltage values. For larger values of
the electric field, the magnification due to ionization
by collision becomes increasingly large and a limiting
value is reached when the voltage is such that the ionization becomes cumulative and a continuous glow discharge
takes place. This effect makes the gas-filled cell more
sensitive; for stable operation however, the cell should
be operated well below the glow voltage, and always connected in series with a high resistance. It has been found
24.
I
r
that the glow discharge, if allowed to pass for a short
time, increases considerably the sensitivity of certain
cells. If maintained for a long time, the glow discharge
damages and eventually destroys the photoelectric property of the cell. This makes the use of gas-filled cells
always a little uncertain, as an excessive illumination,
as well as an excessive electric field, can cause the
glow discharge.
If now, the voltage is kept constant and the amount
of light incident on the cathode of a vacuum cell is in-
I
I
I
creased, it is found that the current given off is a linear function of the illumination. This linear characteristic, in addition to an almost instantaneous response
constant over several thousands of hours of operation,
is a distinct advantage of the vacuum cell.
The gas-filled cell may exhibit a similar linear
characteristic, although in most cases the increase in
sensitivity is obtained at some sacrifice of linearity
of response. The speed of response of the gas-filled cell
I
is limited by the rate of decay of the ionization of the
gas, but it is satisfactory in most applications. The
constancy of operation of a gas-filled cell is probably
not as satisfactory as that of a vacuum cell, as " fatigue " phenomena, not too clearly understood, have been
25.
observed.
1'
As a result of this brief discussion,
it
is apparent
that the increase in sensitivity is practically the only
point of superiority of the gas-filled type of cell, and
the vacuum type should always be used when possible.
I
I
1
I
I
I
Cathode Material
From the study of the photoelectric properties of
different materials, as well as from theoretical considerations, it has been established that each material is
affected by what is called the photoelectric threshold,
which in effect defines a maximum wavelength above which
the photoelectric emission can no longer take place. For
most materials which might be used as a cathode, the
threshold value is shorter than the shorter wavelength
of the visible spectrum, and, as we,have pointed out before, only four metals have been used so far in the visible spectrum, although practically any metal will give
a photoelectric response to ultra violet radiation.
The earlier forms of photocells contained a potassium cathode sensitized by a discharge in hydrogen by the
so called Elster and Gertel process. Subsequent developments however, have shown that the most satisfactory cells
are those which have cathodes consisting of very thin
alkali metal coatings on a suitably prepared base.
26
Various combinations have been investigated with the result that the caesium-on-silver-oxide cathode has proved
most efficient.
The "1mass " layer potassium cathode gives a maximum
spectral sensitivity for a wavelength of about 4400 Angstrom units. The " thin " layer caesium-on-silver cathode
gives a maximum sensitivity for a wavelength of about
7600 Angstrom units. The caesium-on-silver maximum is approximately three times the potassium maximum, in comparable units. In that respect, it appears that the use of
the mercury line of wavelength 5461 Angstroms, is unfortunate as the relative emission of most cathodes at that
wavelength is comparatively low. However, that situation
can be remedied: it is possible to bake the caesium oxide
until the cathode becomes ivory-colored and the sensitivity drops to a low value; if pure argon is then introduced and a glow discharge is established, as during oxidation, it will be found that the surface is re-activated
and the sensitivity returns to its initial value; the gas
may then be removed. A cell prepared in this fashion will
have an excellent sensitivity with its maximum in the
green region of the spectrum.
Dark Current
It has already been pointed out that the voltage
27.,
applied to gas-filled cells should be well below the glow
discharge voltage. In the case of vacuum cells, no gain
is made by increasing the voltage above a value which
gives approximate saturation. Actually, high voltages are
detrimental to satisfactory operation of the cell, as leakage current under such conditions becomes of increasing
importance. In order to minimize the effect of leakage
current, a vacuum cell should be provided with an internal guard ring. Leakage current tending to pass between
the electrodes can then be shunted through the guard ring,
and so avoid passing through the measuring instrument in
the circuit. It is often necessary to reduce external
leakage by the use of an external guard ring.
In every photocell, a small current passes when the
cell is not illuminated. This current is due not only to
the leakage current but also to a cathode emission similar to that produced by a small illumination. This is
usually termed " dark current ". It is obvious that if
the illumination under consideration is small, the dark
current may be very troublesome, to the point of making
measurements meaningless. The cell construction most suitable to reduce the dark current to a minimum is that in
which the electrodes consist of a centrally placed anode
of small dimensions in the center of a spherical surface,
28.
the whole of which forms the cathode.
Unfortunately, the types of thin film cathodes which
are most efficient, have an abnormally large dark current
which is in the nature of a thermionic emission, appreciable even at atmospheric temperatures. In most industrial
applications, where bright lights are used, this dark current is negligible, compared to the photoelectric current.
This is not the case when it is desired to measure faint
illuminations. These considerations emphasize the necessity of using an intense light source and of producing
wide and bright interference fringes. They also complicate further the intelligent choice of the proper photoelectric cell.
Photovoltaic Cells.
Photovoltaic cells are those in which light produces
directly or indirectly an electromotive force.
A number of electrolytic photovoltaic cells have been
developed in the past but they have been overshadowed by
the recent development of " dry " photocells, also known
as " rectifier " cells or " barrier layer " cells. Ever
since the success of Lange in 1930 in obtaining relatively
large currents from the dry photovoltaic cuprous oxide
cell, the photovoltaic cells have come to a new level of
prominenc e.
29.
The cuprous oxide cell however, has one disadvantage in common with the selenium cell: it suffers from a
slow I fatigue " when subjected to high intensity illumination. This is not too important as it " recovers " normality very rapidly, after a brief rest, and no permanent
harm results. On the other hand, a very interesting feature of the dry type of photocell is the relatively large
currents obtained with them. With an orthodox photoelectric cell, 60 micro-amperes per lumen is a very good emission; but for the same illumination, a dry cell will deliver 120 micro-amperes or over twice the current obtainable from a gas-filled photocell.
The dry photocell is not used entirely in place of
the ordinary photoelectric cell, however, the reason being
that it unfortunately possesses a very low internal resistance, which prohibits its use for television or sound
reproduction. It is used wherever practicable, for example
in small pocket photometers.
Another type of dry photocell is the Weston Photronic cell. It consists of a special iron-selenium lightsensitive disk which is both sensitive and permanent. It
uses the barrier-layer principle which does not require
an evacuated space as in the usual forms of photocell.
It has a color response quite similar to that of the hu-
30.
man eye which is a favorable characteristic if a mercury
arc is used as light source. It has, however, a large
electrical capacitance which limits its use at high frequencies. This last point is not a disadvantage in the
present investigation. Since the cell uses no batteries
or any source of external current, there is practically
no dark current. The cell will deliver at least 150 micro-amperes per lumen. Such a high current output makes
amplifying unnecessary in many cases. The Photronic cell,
like most photovoltaic cells, has a straight line relationship between current and illumination if the external resistance is small relative to the cell resistance.
As far as is known, it is not harmed by high intensity
illumination nor is it subject to physical or chemical
change.
Therefore we can say to conclude this discussion,
that the Weston Photronic cell seems to be preferable to
either a vacuum or gas-filled photoelectric cell. The
Photronic cell satisfies all of our requirements. The
photoelectric cells offer probably a more perfectly linear characteristic and a more perfectly instantaneous
response, but it is at the cost of higher dark currents
and a lower sensitivity. The linear characteristic and
31.
the speed of response of~ the Photronic cell will certain-
Ily
be better than our purpose really warrants.
32.
S E C T IO N
V
AMPLIFICATION
AND
SIMPLE RELAY CIRCUITS
33.
AMPLIFICATION
AND
SIMPLE RELAY CIRCUITS
Investigations involving the measurement of photoelectric currents range from those in which the photoelectric effect itself is the center of interest, to those
in which a photoelectric cell is merely a tool for indicating light intensities. The currents may vary in magnitude from 10-15 amperes to 10-3 amperes. They may be constant or may fluctuate rapidly. Hence a wide variety of
methods of measurement has been developed.
The purpose of the use of a photocell is in general,
the measurement, the comparison or the detection of a
number of objects.
Measurements are usually the concern of the physicists.
They favor direct measurements, conveniently made with a
galvanometer if the current exceeds 10- 9 amperes, or with
an electrometer of the Compton, Lindemann or Hoffmann
types if the current is smaller than 10~9 amperes. Currents as small as 10-18 amperes can be measured. It is
even possible to detect the emission of single photoelectrons by means of a modified Geiger-Mfiller tube.
In most photoelectric applications however, such as
photometry, colorimetry, telephotography, sound reproduc-
34.
tion, or television, the purpose of the cell is more to
detect and compare than actually measure changes of light
intensity. This can be done with a much greater measure
of success if the photoelectric current is amplified by
means of thermionic vacuum tubes, or gas-filled relays.
A great amount of work has been done in the field
of current amplification and a number of standard circuits
have been developed in addition to many arrangements used
in specific cases. Small and rapidly varying currents can
be reproduced with great fidelity and amplified by a tremendous factor. In the present investigation, however,
it appears that the amplification of the photoelectric
current will be far from being a critical phase of the
completed apparatus. The neatness of the interference pattern and the reliability of the cell are certainly of more
vital importance.
Just how much amplification is required depends entirely on the use to which the current is going to be put.
In most industrial applications, the use of an ordinary
relay, which is either
"
on " or " off ",
according to
whether the exciting current is above or below a critical
value, is perfectly satisfactory. However, mechanical
relays, even of the best type, have several disadvantages
which restrict their use. Usually they cannot be opera-
Now----
35r
ted by the photoelectric current directly, so that an
initial stage of amplification is required. The electrical contacts after repeated operations deteriorate. They
may not operate instantaneously and hence introduce an
undesirable time lag. Finally the threshold value of the
current at which the relay is supposed to operate is not
sharply defined.
All these objections are eliminated if a Thyratron
is used in place of a mechanical relay. The Thyratron is
essentially a hot-cathode mercury-arc rectifier in which
a control grid has been placed. It is so designed that no
current can pass through until the grid potential is raised above a certain critical value. One simple Thyratron
relay circuit is connected in such a way that control is
obtained by applying an alternating current to the anode.
In that way, the tube acts as an on-and-off relay with a
very high power amplification and with practically no
hysteresis such as a mechanical relay would have. There
are several other Thyratron relay circuits which can give
a very nice control.
The Thyratron is admirably adapted for use as a relay in conjunction with a photoelectric cell of either
the vacuum or gas-filled type. Since it passes a current
of several amperes, it can serve as an excellent relay
36.
which can be controlled directly by the photoelectric
current without preliminary amplification. In the case
of a photovoltaic cell however, no external battery is
used, and the voltage generated by the cell is so small
that the direct Thyratron control cannot be used.
The manufacturers of the Weston Photronic cell, in
view of that fact, have developed a sensitive relay, which
they call Sensitrol. They claim that it can operate on
values down to 2 micro-amperes or 1/2 millivolt. Such a
relay can usually be made to operate directly on the Photronic cell current or voltage. However, some amplification may be necessary in some cases.
It appears therefore, that the Thyratron control is
extremely desirable if a photoelectric cell is used, whereas
the Sensitrol relay or a similar device is better adapted
to use with photovoltaic cells such as the Weston Photronic cell.
V
37.
S E C T IO N
AUTOMATIC
VI
RECORDING
OF
THE
PHOTOELECTRIC
CURRENT
38.
AUTOMATIC
RECORDING
OF
THE
PHOTOELECTRIC
CURRENT
The automatic recording of the photoelectric response can be obtained in several ways.
We can have a continuous recording if we use a selfbalancing potentiometer such as the Leeds and Northrup
Curve Drawing Potentiometer Recorder. This device will
automatically draw a continuous curve showing the variations of the photoelectric current. By calibrating properly the recording paper and by making approximately
constant the speed of motion of the interferometer,
it
should be possible to determine with accuracy the displacement to be measured, without actually having to count
the number of maximum of current as recorded on the paper. This method however,
requires more intricate and
fragile equipment and does not simplify very much the
human labor required.
Another possibility consists in the use of a Veeder
magnetic counter, suitable for applications involving
counting up to 3 or 4 a second. A few words on the operation of the counter may not be out of place:
The cycloneter portion of the counter is actuated
39.
through a link from a crank bolted to the armature spindle
which is supported in bearings attached to the field poles. A side knob is provided so that the counter can be
reset to zero by hand. When the field circuit is clcsed
through the photocell relay contacts, the armature rotates
into the most intense part of the field against the torque of a spring which returns it to the zero position
when the impulse ceases. The rotation of the armature
actuates the connecting link and operates the counter.
Such a counter would clearly have the advantage of
giving a direct and almost immediate answer. The counter
could actually be geared in such a way as to read directly in any desired unit of displacement. In that respect,
it
would be quite satisfactory. It
is apparent,
however,
that if a high accuracy is desired, it is necessary to
measure, or at least be able to estimate, fractions of
the number of light reversals on the interferometer, and
not only integral numbers as would be the case with a
simple magnetic counter.
To do that, it may be necessary to use auxiliary
counters. However, it should be reasonably easy to estimate fractional numbers, if the displacement is made relatively uniform which is not too difficult.
In that connection, it seems that it should be re-
40.
membered that the error found in most testing machines
is from 1/7% to 1%. Therefore little is gained in the
final analysis by making the measurement of strains disproportionately accurate.
The magnetic counter recording is attractive because
it makes for extreme simplicity, although it is probably
somewhat detrimental to accuracy. The continuous paper
recording offers a more tedious but more accurate method.
41.
S E C T I O N
CONCLUSION
VII
42.
CONCLUSION
In the preceding sections, the factors involved in
the selection of the different pieces of equipment have
been discussed.
The following conclusions have been reached:
(1) The mercury arc is probably one of the most
conveniently operated highly intense source of
monochromatic light.
(2) A bright pattern of concentric interference
rings with a large central spot, can be obtained
without the necessity of frequent adjustments,
if the optical system consists of an optical
flat and a lens with a large radius of curvature. A prism may be substituted in place of
the optical flat to increase the brightness of
the pattern and facilitate the design of the
apparatus.
(3) A Weston Photronic cell is likely to be a
satisfactory light-sensitive device. It has a
maximum response at the wavelength of the mercury line used. It is both sensitive and permanent. It has practically no dark current and
can be made to give a linear response if the
43.
external resistance is made small relative to
the cell resistance.
(4) A Thyratron relay circuit is to be recommended if a photoelectric cell is used. If however, a Photronic cell, or any other barrierlayer photocell, is selected, a relay such as
the Sensitrol should be preferred. Amplification
may or may not be necessary.
(5) The relay circuit can be made to operate
a Veeder magnetic counter which automatically
records the electrical impulses originating at
the photocell each time that the central spot
of the interference pattern is bright.
If a
self-balancing potentiometer is used in place
of a magnetic counter, a continuous recording
of the current variations can be obtained.
The above results are intended to be of some use to
future investigators who may work on this problem. It is
clear that the survey that has been done so far is but
a small step toward the solution. It is now necessary to
go through the actual optical, electrical and mechanical
design. However, this will probably have to wait until
the equipment required is available. In this connection,
44.
it is likely that tests will be found to be an indispensable check on the design.
The optical design will be concerned mainly with the
relative disposition of the light source, the interferometer and the cell. It will also involve the determination of the dimensions of the lens and prism and of any
auxiliary lenses or prisms that may be required. The optical design will be influenced by the size of the cell
window and will be continually guided by the conditions
of satisfactory operation of the cell.
The electrical design will deal principally with the
relay circuit and the determination of the critical value regulating the on-and-off action. In addition some
control may be necessary in the light source circuit.
The mechanical design will have to respect closely
the optical arrangement. It may be found necessary to provide a cooling system, if the light source is very hot
and the apparatus compact.
The completed apparatus, if successful, will provide
a means of calibrating strain measuring instruments, such
as Huggenberger Tensometers, with the utmost simplicity
and speed and an appreciable degree of accuracy. The accuracy could be improved upon, if a continuous recording
of the photoelectric response were taken. This could be
45.
done in several ways, all of which would involve some
complication of the procedure. Whether such a complication is warranted can be determined only in specific cases.
The use of such a recording interferometer may not
be limited to the calibration of Huggenberger Tensometers.
It is likely that many other uses could be found for it
in science and industry.
46.
3 E C T I 0 N
APPENDIX
VIII
47.
APPENDIX
In the following pages, technical information on
Weston Photronic cells and Sensitrol relays taken from
this Company's Bulletins B-19-B and B-20-A, will be found.
Also shown are two models of Veeder magnetic counters.
It is hoped that this information may prove to be
of some use to future investigators.
48.
T ECHNICAL
DATA
ON
WESTON
PHOTRONIC
CELLS
e
1
THE PHOTRONIC CELL
The Photronic cell is a self-generating "Solid" or so-called
"Barrier Layer" type photoelectric cell. This cell is distinguished from other types in that the semi-conducting medium
between its electrodes consists of a dry solid, whereas the semiconducting medium in phototubes is a partial vacuum or gas,
and in photovoltaic cells is a liquid. The light-sensitive selenium material used in Photronic cells is mounted between two
metallic electrodes, the positive electrode serving as a rigid
base and the semi-transparent negative electrode serving as an
electron-current collecting surface.
Photronic cells are entirely electronic in their action, the
energy of the impinging light radiation being converted directly
into electrical energy without altering the structure of the cell
itself. The magnitude of the current developed by the cells, for
levels of illumination normally encountered, is sufficient to
operate direct-current instruments and relays directly, without
amplification or other auxiliary equipment. Details on cell characteristics, applications and use will be discussed later.
Photronic Cells have been made in three types, differing
onlyin details of construction and performance characteristics.
The Type 1 Cell is of medium output and is the most rugged.
It is capable of withstanding prolonged high intensity illumi-
nation but has fatigue characteristics which are sometimes
troublesome.
The Type 2 Cell had high output per unit of light intensity
and has been superseded by the Type 3.
The Type 3 Cell is the result of continued research and
development. It not only has the same high current sensitivity
as the Type 2 but also has less fatigue and greater permanence.
It approaches the Type 1 Cell in its ability to withstand high
intensity illumination and surpasses the Type 1 in sensitivity
and freedom from fatigue.
The Type 1 cell is recommended for use where the illumination exceeds 600 to 800 foot-candles over prolonged periods
because of its ruggedness and the fact that high sensitivity is
not required. The Type 3 cell is recommended for general use,
such as for light, color or turbidity measurements and in particular (because of its greater sensitivity) for measuring low
light intensities. In order to provide cells having a high degree
of uniformity, the Type 3 cells are individually tested, grouped,
and listed according to output and linearity. Cells specially
selected for spectral uniformity, and paired cells matched for
output and linearity, are also available as listed on page 12.
49.
Ti
CELL SPECIFICATIONS & ORDERING INFORMATION
CELLS FOR GENERAL USE
TYPE 3 CELLS. The following table and curves in Figure 7
summarize data on five selected groups of Type 3 Photronic
cells recommended for general use. Average values of output
and of linearity, and limits within which they are selected, are
given along with their color code. Each cell is color coded by
means of colored dots on the front edge of the case. Threaded
type terminals are standard on Type 3 cells.
TABLE 3
(a)
Approx.
Microamperes
per Foot-Candle
Model
4.4 ± .3
594RR
4.0 ±L .3
594YR
3.5 -- .5
594GB
2.9 ± .5
594BB
2.6 ± .3
594YY (c)
(b)
Approx.
Color
Prices
Code
$14.00
Red-Red
12.00
Yellow-Red
10.00
Green-Blue
8.00
Blue-Blue
Yellow-Yellow 12.00
Linearity
Factor
.86 ± .04
.86 ±: .04
.84 ±- .09
.84 ± .09
.92 ± .03
(a) The approximate current output and limits of selection are for a
light intensity of 20 foot-candles and an external circuit resistance
of 200 ohms.
(b) The approximate linearity factors and limits of selection represent the ratio of output per foot-candle at 200 foot-candles to that
at 20 foot-candles for an external circuit resistance of 200 ohms.
(c) Specially selected for use at high illumination and exposure to
North sky.
SPECIAL CHARACTERISTICS. Cells can be furnished selected for similarity in spectral sensitivity, matched for linearity,
or matched in pairs for output and linearity as listed below.
When ordering selected or matched cells, it is advisable to
mention the approximate range of light intensity and external
resistance that will be encountered.
Symbol
L
0 (e)
S (f)
V (g)
W
Surcharge
Special Characteristics
described
as
2%
within
Linearity matched
$ 3.00
in Table 3 --------------------------------------------Output and linearity matched in pairs within
5.00
2% as described above-....................................
7.50
Spectral selection ...............................................
7.50
Viscor Filter equipped.......................................
15.00
.....
Weather-proof housing ..........-.................
(e) Limits of selection for a large number of cells will depend upon
the quantity involved and the requirements of the application.
(f) All photoelectric cells differ somewhat in spectral sensitivity.
Cells spectrally selected will be within 1/5 of the normal spectral
variation.
(g) When Viscor Filter equipped, the cell output is reduced to approximately 40% of the normal cell output for 2700*K. light.
TYPE 1 CELLS. Only one group of Type 1 cells is carried in
stock. These cells have a current sensitivity of 1.6 ± .5 microamperes per foot-candle and a linearity of .75 or more for
standard test conditions defined above. These cells are not
color coded. They list at $10.00. Prong type terminals are
standard on Type 1 cells.
HOUSING.
Cells are normally housed in a black bakelite
case 1" deep and 2
" in diameter. Type 3 cells are regularly
supplied with threaded binding post type terminals, and a
threaded back for mounting purposes whereas Type 1 cells are
regularly supplied with prong type terminals. Either type terminal will be supplied on request at no extra cost. Cells can be
supplied in a 3 " weather-proof metal case provided with
" pipe connection for conduit wiring as listed above.
standard
The weather-proof housing is recommended where cells will be
subjected to abnormal humidity, corrosive fumes, etc. Cells are
normally provided with a protecting window of glass, but when
required Viscor, quartz or other special filter glasses can be
supplied in place of the regular glass window at extra cost.
Example of the use of the above symbols:
ORDERING.
Model 594GB-OS indicates that Type 3 cells having an output
of 3.5 microamperes, matched for output and linearity, and
selected for spectral uniformity at a list price of $22.50, are
desired.
CELLS FOR USE WITH RELAYS
Differently
colored back plates identify Type 3 Photronic
cells for relay use. These cells are carefully selected for characteristics most suitable for such application. To properly order
and identify relay cells which are also rated at 20 foot-candles
and 200 ohms external resistance, consult the following table.
Model
Approx. Microamperes
per Foot-Candle
* 594R
* 594B
594Y
4.5
3.5
2.5
Identification
Red Back Plate
Blue Back Plate
Yellow Back Plate
Selected for highest possible output. Minimum output is stated
but usually will be found higher.
For relay applications not involving skylight and where highest
sensitivity is not required.
Processed for relay applications where the cell faces the sky or
sun. This cell will operate under continuous exposure to North
sky but a water or glass filter is recommended to reduce the
heating effect if continuously exposed to direct sunlight.
PHOTRONIC CELL MOUNTINGS
UI
With
Threaded Terminals
ho
With
Pron q Terminals
In Bakelite Case
Prices
$15.00
10.00
10.00
Single Cell
Three Cell
In Weatherproof Housing
50,
411111I
IZFVV
250
1 1
1
1 1 1
1 1 1
EXT. RES. OHMS
COLOR CODE
LINEARITY FACTOR
Rflfl i i i i i i i i i i i
.90
EXT. RES.
}RR
-HMS
#OH
225
100
4-
20G
05
.90
700
300
2?
.821
...------.9 3
500
600
700
Y
175
91
w
1000
150-
< 500
-
0
w
95KBBI
ljYY
Lii
.75
Lii
a-
z 400
Ix
0
z
-
z
1500
w
2000
U 300
3000
16RR
3
10
4
5
0/
0
00
30
20
10
ILLUMINATION IN FOOT
5t
40
CANDLES
Fig. 4-Effect of Illumination and External Resistance
on Current Output of Type 3RR Cell.
(Tungsten lamp at 2700*K.)
I
IAA
RE S.
IEXT. MS
30003
10
20
40
--
2500 ---
/
U)
a-
2000-
-60
80
100
1500
Lt4
/TI/V/,____
150
__
Fyru10ff
u
500
:200
fi
500
.1000,
1500
4BBI
1.W
100
I
25
I
.8A 6YR
.8A4GB "
z
III
200
I
0
2000
ILLUMINATION IN FOOT CANLSL
Fig. 5-Effect of Illumination and of External Resistance
on Current Output of Type 1 Cell.
(Tungsten lamp at 2700"K.)
7q
50
100
ILLUMINATION IN FOOT
150
CANDLES
200
51.
ATTAGE OUTPUT. When cells are used with r
hich require a relatively high torque for good contact,
inear output characteristic is less important than the maximum
wattage output obtainable from the cell. Cells for relay use
are, therefore, not normally selected for linearity, but are
selected for high internal leakage resistance which, in addition
to current sensitivity, determines the relative efficiency of the
cell as an electrical converter of radiant energy. The cells
selected for relay use are listed on the basis of current sensitivity as shown on page 12.
The curves in Figure 14 show the microwatts output for a
type RR cell for various values of external resistance and illumination. The values of maximum wattage output were taken
from these curves and are plotted in Figure 15 against illumination as a log-log function.
The result is nearly a straight line relationship. It is useful
in determining the external resistance for maximum power
output at any given illumination.
rL
F
350
10*Ct
300C
300
-
--
-
-40Co
250
z
w
U
0
LL..
w
600
ol
x 150
0
400
RFOOT
O
CANDLES
w
-j
w 100
150- /
100
80
1
-
-
I
-
-
-
-
_
I
00
60
110
40
0
0--T-
301
20
20
1
-
50--------------__
50-
500
x 210
U
2
820
L
6
MAXIMUM WATTAGE
OUTPUT CURVE.
4
3
0
3000
2000
1000
EXTERNAL RESISTANCE
40000
IN OHMS-
Fig. 14-Wattage Output of Type RR Cell at Various Values of
Illumination and External Resistance.
4UUU
3000
-----
CL
2000
0
LID
0-
1000
7
600 -
0
LL
n 400 -
o 300 Z
u3
200-
100
20
30 40
60 80 100
200
300 400
600 800
ILLUMINATION IN FOOT CANDLES
ig 15-External Resistance
L9.ternal
for Maximum Wattage
Type RR Cell.
Output
4
Fig.
200
150
100
50
ILLUMINATION IN FOOT CANDLEES
Force of Type 3GB CeA
(Tungsten lamp at 2700*K.)
l1-Electromotive
_
52.
H
IE _.~
t
L
'ci
I
I
I
ILLUMINATION
I
PERMANENCE. Experience indicates that Photronic
have a long life if thoroughly protected against unfavorable
external influences-particularly moisture, heat, corrosive
fumes, and excessive applied voltages. When damage occurs,
it is usually traceable to some outside influence other than illumination itself. Many cells that have been in use for nearly ten
years are still functioning satisfactorily.
r-
I
20 FOOT-CANDLES
27
oK0 j
1
EXT.
N
As a rule, the
the Type 3 where
prolonged periods
high sensitivity of
RES
zoo
Type 1 cell is recommended in preference to
the illumination may exceed 600-800 f.c. over
because it is more rugged and because the
Type 3 cells is not required.
100--0
---
40-
0
0
APPLIED EXTERNAL VOLTAGE. The cell surface is not designed for carrying large current densities; hence, cells must
not be connected in circuits in such a way that damage from
burning will result from external voltages. Here also the possibilities of high voltage arising by induction must not be overlooked.
z
z4
w
t 00
0-n
{[j
-1.
-103
20
10
30
30**
40
MOISTURE EFFECTS. Experience has shown that moisture
has a harmful effect upon the cell. It may permanently damage
a cell by promoting destructive electrolytic action between the
constituent parts or between the cell and its external connections. It also lowers the effective internal resistance and consequently, reduces the current delivered to the external circuit.
Where cells are mounted outdoors or subjected to moist conditions indoors, weather-proof housings are advisable.
TEMPERATURE IN DEGREES CENTIGRADE
Fig. 17-The Effect of Temperature on Current Output of Type 3
Cells at 20 Foot Candles Illumination.
'I
+ 1In
ILLUMINATION
2OOFOOT-CANDLES
2TOO* K
EXT.
RES.
0
N~
z
300
0
200
Radiation from tungsten filament sources contains a large
proportion of infra-red radiation. When cells are used to
measure high intensities, say 500 f.c., or more, from such
sources, or from sunlight, they should be exposed only long
enough to obtain readings unless protected by a water or other
special infra-red filter.
EXT.
RES.
-------
'a
0
INFRA-RED OR HEAT ABSORBING FILTER. In order to protect Photronic cells against heating, an Aklo filter made of solid
green tinted glass manufactured by the Corning Glass Company, can be supplied instead of the regular clear glass window. It removes a large portion of the infra-red radiation found
associated with light. Since infra-red radiation produces no
visibility and very little cell response, but is very effective in
producing heat, it is often desirable to remove it from the light
beam before it reaches the cell disc. It is preferable to mount a
heat absorbing filter outside the cell when possible because the
filter may become quite hot and transfer its heat by conduction
and secondary radiation to the cell disc if mounted in the cell.
0.
sw
z
z
I-
4--
-10 --
7*WE
-
U
The Aklo filter alters the effective spectral sensitivity
call and, therefore, requires special correction factors
used for measuring light from different sources. Its total
mission to 2700'K. light, measured as cell response, is
65%.
-o
150
10
20
30
40
TEMPERATURE IN DEGREES CENTIGRADE
Fig. 18-The Effect of Temperature on Current Output of Type
Cells at 200 Foot Candles Illumination.
SAFE TEMPERATURE. Photronic cells should be protected
against being heated to a temperature beyond 60'C. (140*F.)
since continued heating may result in permanent changes in
sensitivity. The cell surface absorbs infra-red or heat rays and
unless care is taken, may become heated to a greater degree
than is indicated by the ambient temperature or that of the
outer case.
3
FATIGUE. Photronic cells have no dark current or drift and
are not subject to erratic or non-reversible changes. However,
upon an initial exposure to a luminous source the current output
tends to fall off slightly with exposure so that a short time interval may be required before the cell current reaches a constant
value. Therefore, when relatively high accuracy is desired, a
minute or so wait is recommended before taking readings.
of the
when
transabout
SPEED OF RESPONSE. The cell response is so much more
rapid than that of meters and relays used with them that it
may be considered instantaneous for such use. Actual tests
have demonstrated that the cell response is sufficiently rapid
to record the passage of a rifle bullet through a beam of light
incident upon the cell.
LIGHT
E
rSEMI-TRANSPARENT-
-X
Individual cells differ in the amount of fatigue they exhibit.
CATHODE
-The Type 3 cells have less fatigue than the Type 1 cells and
t
LC
SEMI-CONDUCTOR .x
some may have zero fatigue or even a positive fatigue, that is,
I
an increase in current output. However, a decrease in output of
1 or 2% between
and 20 minutes of illumination at 100 foot
METAL BASE
~
candles and low external resistance is normally to be expected.I___------------igher values of illumination and external resistance tend to
ncrease the apparent fatigue primarily because of temperature1-Cross
Sectional View of Photronic Cell Iut
es produced in the active surface of the cell.
Cell Action. (Not to scale.)
I
53.
MODEL 721 PHOTOELECTRIC
if
LMODEL
POTENTIOMETER
The Model 721 photoelectric potentiometer is fundamentally a d-c amplifier fo
the actuating of a 10-milliampere instrument, recorder or relaying device, or a
group of such devices in series, provided their total resistance is not over 1,000
ohms. The gain is such that 10 milliamperes output can be had with inputs as low
as 5 microamperes or 2 millivolts.
The instrument functions as a potentiometer in that it takes no energy from the
input circuit, since, in a manner similar to a potentiometer recorder, the input energy
is balanced out.
The device combines a sensitive mirror galvanometer, light source, photo tubes
and vacuum tubes for output, the output energy being fed back to oppose the
input. All power for operation is supplied from an a-c line, less than 40 watts
being required at 60 cycles 110 volts.
The amplified output of the Model 721 is truly linear in terms of the input and
instruments in the output circuit may be calibrated in terms of the primary quantity,
which in turn may be temperature applied to a thermocouple, light on a photo cell,
and the like.
Measurement of extremely small currents or voltages are easily possible with
Model 721, and instruments or recorders can be operated at high speeds. Balancing is continuous and response to change is virtually instantaneous. Descriptive
bulletin available upon request. Correspondence invited.
otoelectric Potentiometer
721
Model 721 Photoelectric Potentiometer with two range standards and
Price $
-------------------------Model 301 Indicator (portable or switchboard mounting)--
S.PAT.
T$.NT
VO
RO3S
A
M DEiMODEL
705
SigeFxe
otctobeAdjustable
[Single
29
Contact
SSENSITROL
MODEL 705
Fixed Contact with
table Index andl Solenoid
RELAYS-MODEL 705
Weston Sensitrol Relays are the first commercial types combining high sensitivity and high contact capacity. Positive operation is obtained on values as low as
1 millivolt or 2 microamperes d-c; and 0.5 volt or 10 microamperes a-c with an
external rectifier. Contact capacity is 5 watts at 110 volts.
Operation is as follows: The stationary contact is a small powerful permanent
magnet and the movable contact is an iron "rider" mounted on the pointer which
travels over the relay scale. The operating torque moves the pointer into the field
of the stationary contact. This contact then draws the movable contact in and holds
it firmly. Perfect contact is assured and chattering is eliminated. Contacts remain
closed until reset manually or with an electrical solenoid reset. The latter is mounted
on the base of the relay and is arranged with 2 small arms which mechanically
push the contact arm away from the contact when the solenoid is energized. Accuracy in general may be considered as within 5% of the range in Types 1, 2 and 3.
Types 4, 5 and 6 may be considered accurate within 10% of the range. When
special characteristics are required these accuracies may not apply. See typical
scales and dimensional diagrams on pages 10 and 11.
Description
of Types
Type i--Single Fixed Contact. Stationary contact remains in a fixed position. Can
be supplied to make contact on increasing or decreasing values. Reset from front
or back.
Type 2--Single Adjustable Contact. Stationary contact can be adjusted to any
desired operating value over either lower or upper half of scale. Provided with a
movable index which indicates operating point. Adjusted by means of knob on
front of case. Reset from front only.
Type 3--Double Adjustable Contact. Has two adjustable stationary contacts,
one
for high and one for low values. Provided with two indexes for indicating high and
low operating points. Reset from front only. Closest that contacts can be set
together at center of scale is approximately 20 % of scale range.
Type 4-Single Fixed Contact with Solenoid Reset. Supplied to make contact on
increasing or decreasing value. Solenoid is mounted permanently on back of relay,
extending approximately 2 ". Designed for 6 volt d-c intermittent service. Can be
supplied for any d-c or 60 cycle service.
Type 5--Single Fixed Contact with Solenoid Reset and Adjustable Index. Similar to
Type 4 but equipped with adjustable index to indicate operating point.
the
index shifts the control spring similar to the action of a zero corrector.Turning
Has two
scales; one for setting index, the other for reading pointer position.
IType
6-Double Fixed Contact with
increasing and decreasing values.
When
Solenoid
Reset. Makes contact on both
ordering Model 705 Relays, please use type numbers printed above
and specify whether contacts are to close on increasing or decreasing values.
55.
MAGNETIC
COUNTER,
FORM UD
Aq
DIRECT CURRENT ONLY
SPECIFICATIONS
THESE counters are practically the same, in action and operation,
as Form US Counters, except that they will operate satisfactorily
only on direct current. Thus they may be connected to dry cells
or storage batteries, as well as to generated current.
DIMENSIONS:
24" long, 3" wide, 5 7/32" high.
FIGdRES:
.166" high.;White on black background.
An eighty ampere-hour storage battery, with the time of contact
of commutator or circuit-breaker adjusted to the minimum required, should run the small Form UD Magnetic Counter to
5,000,000 counts on one charge.
SPEEDS:
700 counts per minute.
RESET OR NON-RESETP Reset.
LuBRICATION:
Form UD Counters can be furnished for the following voltages,
6, 12, 24, 55, 110, 220.
Not required.
I
II
FINiSH:
Black shrivel.
WHEN ORDERING:
II.'a momm
Specify Voltage.
INaiA
-R 00T
A'
56o
MAGNETIC COUNTER,
r
FORM US
ALTERNATING CURRENT ONLY
moo,="
by electrical energy
counters may be placed at any distance
the count is to be taken - a feature which per
er of
counters to be mounted together at a central point. Where articles
to be counted are very small, thin, or light in weight, these counters
(if furnished with coils to suit) can be connected in shunt or in series
with the device.
Form US Counters have stator cores of special dynamo electric
iron, equipped with shading coils. Rotor shaft is fitted with an
inertia governor. These features permit the use of alternating current of the following voltages: 55, 110, 220 (50 to 60 cycles). At
extra charge, counters can be furnished for other voltages from 12
to 550.
Power consumption is approximately 15 watts irrespective of
voltage. The voltage variation permissible is 10%.
Combined registers may be supplied on this magnetic base, see
page 20 for type referred to. Can furnish non-reset registers 6 to
8 figures, see page 18.
ACTUATED
For remote indication
of machine operations.
SPECI FICATIONS
DIMENSIONS:
24' long, 3" wide, 5739
No. FIGURE
5 only.
SPEEDS:
600 counts per minute.
RESET OR NON-RESET?
Reset.
LUBRICATION:
Not required.
FINISH:
Black shrivel.
WHEN ORDERING:
Specify Voltage.
L
N4 AaR0.0T
high.
WHEELS:
I
57.
S E C T IO
N
BIBLIOGRAPHY
IX
58.
BIBLIOGRAPHY
Physical Optics
R.1.V7ood.
Principles of Optics
A.C. Hardy and F.H. Perrin.
Light for the Student
F. Edser.
The Theory of Light
T. Preston.
Cours de Physique G6n6rale
H. Olivier.
Interf6rences Lumineuses
C. Fabry.
Applications of Interferometry
W.E. Williams.
Photoelectric Cells
N.R. Campbell and
D. Ritchie.
Photoelectric Cell Applications
R. C. Walker and T. Lance.
Photocells and their Application
V.K. Zworykin and
E.D. Wilson.
Photoelectric Phenomena
A.L. Hughes and
L.A. DuBridge.
La Cellule Photo6lectrique et ses Applications
Conf6rences d'Actualit6s Scientifiques et
Industrielles. 1929
L. Dunoyer.
History of Photoelectric Cells
E.W. Cowan.
Electrical Enginnering Seminar,Mass. Inst. of Tech. 1942
Electrical Engineering Thesis
T.B. Perkins.
Massachusetts Institute of Technology
1929
Electrical Engineering Thesis
F.B. Parks and B. Poehler
Massachusetts Institute of Technology
1934
Photocell Multiplier Tubes
C.C. Larson and H. Salinger
Review of Scientific Instruments 1940. Vol.11.p. 226
59.
Sensitive Light Meter with Electronic
Amplification
Photovolt Corporation.
Review of Scientific Instruments
1941. Vol.12. p.39
Counting Automobiles on Ambassador Bridge
Electronics
June 1930.
New Fields for Magnetic Contact Relays
Electronics Dec. 1940
A.H. Lamb (Weston
Elect. Instr. Co.)
Applying Magnetic Relays for Sensitive Control A.H.Lamb (Weston
Electronics Feb. 1941
Elect. Instr.Co.)
A Recording Photoelectric Color Analyzer
A.C.Hardy
Journal Opt. Soc. of America, 1929. Vol.18. p.96
Relays and Photronic Equipment
Weston Elect. Instr. Co.
Catalog 1941.
Counting Devices
Veeder-Root Inc.
1940. Catalog No G 40.
Hot-cathode Thyratron
Gen. Elect. Review.
A.W. Hull.
1929. Vol.32. pp.213 and 390.
Characteristics of the
Huggenberger Tensometer
A.S.T.M. Proceedings
R.W. Vose.
1934.Vol.34.
Part II p.
862