Test Instruments & Measuring Devices
The resistance of a wire or a circuit is measured by an ohm meter. An ohm meter
aids the troubleshooter in determining if a ground or a short exists in a circuit.
STATE the electrical parameters measured by each of
the following in-place measuring devices:
EXPLAIN how the following electrical test equipment
and measuring devices are connected to a circuit:
The ohm meter is an instrument
Figure 9 Simple Ohm Meter Circuit
used to determine resistance. A
simple ohm meter (Figure 9)
consists of a battery, a meter
movement calibrated in ohms, and
a variable resistor.
Ohm meters are connected to a
component which is removed from
the circuit as illustrated in Figure
9. The reason for removing the
component is that measurement of
current through the component
determines the resistance. If the
component remains in the circuit,
and a parallel path exists in the
circuit, the current will flow in the path of least resistance and give an erroneous reading.
Ro, in Figure 9, is an adjustable resistor whose purpose is to zero the ohm meter and correct for
battery aging. It is also a current-limiting resistor which includes the meter resistance Rm.
Zeroing the ohm meter is accomplished by shorting the ohm meter terminals ab and adjusting
Ro to give full-scale deflection.