Copyright Goodheart-Willcox Co., Inc.
232 Engineering Fundamentals
Control Components
Control devices direct and/or limit current
fl ow so a circuit meets its desired function. For
example, insulators keep current in the conduc-
tor and protect against shorts, and resistors limit
current fl ow to protect sensitive components.
Insulators
Materials with extremely high resistance that
do not conduct electricity under normal circum-
stances are insulators. The most commonly used
insulators are plastic, glass, paper, rubber, and
mica. Insulators are used to keep electricity
confi ned to the desired circuit path and away from
people and other parts of the circuit. A short circuit
can be defi ned as an undesired path to ground. In
a short circuit, electricity is able to neutralize its
charge without having to fl ow through the load.
Without the resistance provided by the load,
current increases dramatically, which can cause a
fi re or other damage.
Meters in Electrical Engineering
Electrical engineers must be competent in the
safe and proper use of a wide variety of tools and
meters in order to design, build, and troubleshoot
electrical circuits and equipment.
For practice reading and using ammeters,
voltmeters, ohmmeters, and continuity testers,
see Workbook Activity 11-8.
Ammeter
Ammeters measure current flow in a circuit in
amperes. The circuit must be disconnected so the
ammeter can be wired in series and become part of
the circuit. Ammeters must be wired in series with the
proper polarity. Turn the power off, hook up the meter,
and then turn the power back on. Never connect the
meter into the circuit with the power on. Never con-
nect an ammeter in parallel with a circuit. Doing so
can provide an uncontrolled amount of current to the
meter, causing damage to the meter and the circuit
or bodily harm to the person holding the meter. When
measuring an unknown current, always start on the
highest range and work your way down until you get
an accurate reading. Measuring values that are too
high for the range you are on can damage the meter.
Tools
Voltmeter
Voltmeters measure potential differences from
one point to another. Voltmeters are always wired
in parallel with a device in the circuit. This could be
a load or the power source. Always turn the power
off, connect the voltmeter, and then turn the power
back on before taking a reading. When measuring an
unknown voltage, always start on the highest range
and work your way down until you get an accurate
reading. Measuring values that are too high for the
range you are on can damage the meter.
Ohmmeter
Ohmmeters are used to measure resistance
in ohms. Always turn off the power and isolate the
part to be tested. Connect the ohmmeter in series
with whatever you plan to test. The ohmmeter has its
own power supply and tests resistance by applying
a voltage to the item to be tested. When measuring
an unknown resistance, always start on the highest
range and work your way down until you get an accu-
rate reading. Measuring values that are too high for
the range you are on can damage the meter.