Voltmeter usage
PARTS AND MATERIALS
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Multimeter, digital or analog
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Assorted batteries
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One light-emitting diode (Radio Shack
catalog # 276-026 or equivalent)
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Small "hobby" motor, permanent-magnet type
(Radio Shack catalog # 273-223 or equivalent)
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Two jumper wires with "alligator clip"
ends (Radio Shack catalog # 278-1156, 278-1157, or
equivalent)
A multimeter is an electrical
instrument capable of measuring voltage, current, and
resistance. Digital multimeters have numerical
displays, like digital clocks, for indicating the quantity
of voltage, current, or resistance. Analog
multimeters indicate these quantities by means of a moving
pointer over a printed scale.
Analog multimeters tend to be less expensive
than digital multimeters, and more beneficial as learning
tools for the first-time student of electricity. I strongly
recommend purchasing an analog multimeter before purchasing
a digital multimeter, but to eventually have both in your
tool kit for these experiments.
CROSS-REFERENCES
Lessons In Electric Circuits, Volume
1, chapter 1: "Basic Concepts of Electricity"
Lessons In Electric Circuits, Volume
1, chapter 8: "DC Metering Circuits"
LEARNING OBJECTIVES
ILLUSTRATION
INSTRUCTIONS
In all the experiments in this book, you
will be using some sort of test equipment to measure aspects
of electricity you cannot directly see, feel, hear, taste,
or smell. Electricity -- at least in small, safe quantities
-- is insensible by our human bodies. Your most fundamental
"eyes" in the world of electricity and electronics will be a
device called a multimeter. Multimeters indicate the
presence of, and measure the quantity of, electrical
properties such as voltage, current, and
resistance. In this experiment, you will familiarize
yourself with the measurement of voltage.
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, and the black test probe to the negative (-)
side of the same battery. 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 high-range 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.
What happens if you only touch one meter
test probe to one end of a battery? How does the meter have
to connect to the battery in order to provide an indication?
What does this tell us about voltmeter use and the nature of
voltage? Is there such a thing as voltage "at" a single
point?
Be sure to measure more than one size of
battery, and learn how to select the best voltage range on
the multimeter to give you maximum indication without
over-ranging.
Now switch your multimeter to the lowest DC
voltage range available, and touch the meter's test probes
to the terminals (wire leads) of the light-emitting diode
(LED). An LED is designed to produce light when powered by a
small amount of electricity, but LEDs also happen to
generate DC voltage when exposed to light, somewhat like
a solar cell. Point the LED toward a bright source of light
with your multimeter connected to it, and note the meter's
indication:
Batteries develop electrical voltage through
chemical reactions. When a battery "dies," it has exhausted
its original store of chemical "fuel." The LED, however,
does not rely on an internal "fuel" to generate voltage;
rather, it converts optical energy into electrical
energy. So long as there is light to illuminate the LED, it
will produce voltage.
Another source of voltage through energy
conversion a generator. The small electric motor
specified in the "Parts and Materials" list functions as an
electrical generator if its shaft is turned by a mechanical
force. Connect your voltmeter (your multimeter, set to the
"volt" function) to the motor's terminals just as you
connected it to the LED's terminals, and spin the shaft with
your fingers. The meter should indicate voltage by means of
needle deflection (analog) or numerical readout (digital).
If you find it difficult to maintain both
meter test probes in connection with the motor's terminals
while simultaneously spinning the shaft with your fingers,
you may use alligator clip "jumper" wires like this:
Determine the relationship between voltage
and generator shaft speed? Reverse the generator's direction
of rotation and note the change in meter indication. When
you reverse shaft rotation, you change the polarity
of the voltage created by the generator. The voltmeter
indicates polarity by direction of needle direction
(analog) or sign of numerical indication (digital).
When the red test lead is positive (+) and the black test
lead negative (-), the meter will register voltage in the
normal direction. If the applied voltage is of the reverse
polarity (negative on red and positive on black), the meter
will indicate "backwards."
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