The true voltage is found as follows:
no warm-up is necessary. It is a portable, versatile
meter that is free of calibration errors caused by aging
E=
82 - 22
+ 82
tubes or line voltage variations.
22 10 - 1
The VOM does have two disadvantages:
82
1. The VOM can load the circuit under test.
2. The meter movement is easy to damage
E = 117.7 volts
because of improper testing procedures.
MULTIMETER
The VOM (fig. 2-4) includes all the necessary
During troubleshooting, you will often measure
switches, jacks, and additional devices arranged in a
voltage, current, and resistance. Rather than using
compact, portable case. It uses one meter movement.
three or more separate meters for these
The
permanent-magnet,
moving-coil
meter
measurements, you can use the multimeter. The
mechanism responds only to direct current. To adapt
multimeter contains circuitry that allows it to be a
this mechanism for measuring alternating current
and voltages, a rectifier or thermal converter must be
multimeter is often called a volt-ohm-milliammeter
used.
(VOM).
The use of a rectifier affects the calibration of the
One advantage of a VOM is that no external
meter. Waveforms of various shapes have
power source is necessary for its operation. Therefore,
Figure 2-4.--Schematic diagram of a typical multimeter.