2.31 Ohmmeter, Ammeter and Voltmeters
We have talked about potential difference, current, and resistance I think it’s about
time talk about how me measure these quantities.
This simplest device is called a d’Arsonval galvanometer. This is a pivoted coil of fine
wire placed inside a magnetic field. There is a spring attached to the coil in such a way
that with no current, the pointer reads zero.
When there is a current in the coil, the magnetic field exerts a torque on the coil (that is
proportional to the current) which in turn moves the pointer, the spring exerts a
restoring torque that is proportional to the angular displacement.
It is the difference in internal connections that determine if we measure voltage,
current, or resistance. In a circuit diagram, the ammeter or voltmeter is indicated by a
circle with a V or A or a  inserted in the middle.
Ammeter
A device that measures a current is usually called an ammeter. It always measures the
current passing through it (an theoretical ammeter would have zero resistance). We can
adapt the ammeter to measure currents that are larger than its full scale reading by
connecting a resistor in parallel with it (called a shunt resistor).
INSTRUCTIONS
Current is the measure of the rate of electron "flow" in a circuit. It is measured in the
unit of the Ampere, simply called "Amp," (A).
The most common way to measure current in a circuit is to break the circuit open and
insert an "ammeter" in series (in-line) with the circuit so that all electrons flowing
through the circuit also have to go through the meter. Because measuring current in
this manner requires the meter be made part of the circuit, it is a more difficult type of
measurement to make than either voltage or resistance.
Some digital meters, like the unit shown in the illustration below, have a separate jack
to insert the red test lead plug when measuring current. Other meters, like most
inexpensive analog meters, use the same jacks for measuring voltage, resistance, and
current. Consult your owner's manual on the particular model of meter you own for
details on measuring current.
When an ammeter is placed in series with a circuit, it ideally drops no voltage as
current goes through it. In other words, it acts very much like a piece of wire, with very
little resistance from one test probe to the other. Consequently, an ammeter will act as
a short circuit if placed in parallel (across the terminals of) a substantial source of
voltage. If this is done, a surge in current will result, potentially damaging the meter:
Voltmeters
A voltage-measuring device is called a voltmeter. It always measures the potential
difference between two points. An ideal voltmeter would have infinite resistance, so
connecting it between any two points in a circuit would not alter any of the currents.
INSTRUCTIONS
Voltage is the measure of electrical "push" ready to motivate electrons to move through
a conductor. In scientific terms, it is the specific energy per unit charge, mathematically
defined as joules per coulomb. It is analogous to pressure in a fluid system: the force
that moves fluid through a pipe, and is measured in the unit of the Volt (V).
Your multimeter should come with some basic instructions. Read them well! If your
multimeter is digital, it will require a small battery to operate. If it is analog, it does not
need a battery to measure voltage.
Some digital multimeters are autoranging. An autoranging meter has only a few
selector switch (dial) positions. Manual-ranging meters have several different selector
positions for each basic quantity: several for voltage, several for current, and several
for resistance. Autoranging is usually found on only the more expensive digital meters,
and is to manual ranging as an automatic transmission is to a manual transmission in a
car. An autoranging meter "shifts gears" automatically to find the best measurement
range to display the particular quantity being measured.
Set your multimeter's selector switch to the highest-value "DC volt" position available.
Autoranging multimeters may only have a single position for DC voltage, in which case
you need to set the switch to that one position. Touch the red test probe to the positive
(+) side of a battery (or circuit element), and the black test probe to the negative (-)
side of the same battery (or circuit element). The meter should now provide you with
some sort of indication. Reverse the test probe connections to the battery if the meter's
indication is negative (on an analog meter, a negative value is indicated by the pointer
deflecting left instead of right).
If your meter is a manual-range type, and the selector switch has been set to a highrange position, the indication will be small. Move the selector switch to the next lower
DC voltage range setting and reconnect to the battery. The indication should be
stronger now, as indicated by a greater deflection of the analog meter pointer (needle),
or more active digits on the digital meter display. For the best results, move the
selector switch to the lowest-range setting that does not "over-range" the meter. An
over-ranged analog meter is said to be "pegged," as the needle will be forced all the
way to the right-hand side of the scale, past the full-range scale value. An over-ranged
digital meter sometimes displays the letters "OL", or a series of dashed lines. This
indication is manufacturer-specific.
Ohmmeter
This device will measure the resistance across a circuit element.
INSTRUCTIONS
Resistance is the measure of electrical "friction" as electrons move through a conductor.
It is measured in the unit of the "Ohm," that unit symbolized by the capital Greek letter
omega (Ω).
Set your multimeter to the highest resistance range available. The resistance function is
usually denoted by the unit symbol for resistance: the Greek letter omega (Ω), or
sometimes by the word "ohms." Touch the two test probes of your meter together.
When you do, the meter should register 0 ohms of resistance. If you are using an
analog meter, you will notice the needle deflect full-scale when the probes are touched
together, and return to its resting position when the probes are pulled apart. The
resistance scale on an analog multimeter is reverse-printed from the other scales: zero
resistance in indicated at the far right-hand side of the scale, and infinite resistance is
indicated at the far left-hand side. There should also be a small adjustment knob or
"wheel" on the analog multimeter to calibrate it for "zero" ohms of resistance. Touch
the test probes together and move this adjustment until the needle exactly points to
zero at the right-hand end of the scale.
Although your multimeter is capable of providing quantitative values of measured
resistance, it is also useful for qualitative tests of continuity: whether or not there is a
continuous electrical connection from one point to another. You can, for instance, test
the continuity of a piece of wire by connecting the meter probes to opposite ends of the
wire and checking to see the the needle moves full-scale. What would we say about a
piece of wire if the ohmmeter needle didn't move at all when the probes were
connected to opposite ends? Digital multimeters set to the "resistance" mode indicate
non-continuity by displaying some non-numerical indication on the display. Some
models say "OL" (Open-Loop), while others display dashed lines.
Connect the meter's test probes across the resistor as such, and note its indication on
the resistance scale: