connected across a circuit or voltage source to measure
voltage. Panel-mounted voltmeters are permanently
wired into the circuit in which they are to be used.
Portable voltmeters are designed to measure one or
more ranges of voltage. Those intended for
measurement of more than one voltage range are
provided with range selector switches. The range
selector switch internally connects the appropriate
multiplier resistor into the meter circuit for the range of
voltage to be measured; for example, a voltmeter may
be designed to use a O-l milliampere milliammeter as a
voltage indicator. For each setting of the selector
switch, a different multiplier resistor is connected into
the meter circuit. For each selection, a particular
resistor value is designed to limit the current through the
milliammeter to a maximum of 1/1,000 of an ampere (1
milliampere) for a full-scale reading.
In a similar way, voltmeters designed to use a
micrometer, for example, a 50-microampere meter,
include multiplier resistors that limit the meter current
to a maximum value of 50 microamperes. In this case,
50 microamperes are flowing through the meter for a
full-scale deflection of the needle.
Voltmeters that use either a milliammeter or
micrometer to indicate voltage have a scale calibrated to
read directly in volts. The flow of current in either type
of meter represents the electrical pressure (voltage)
between two points in an electrical circuit; for example,
the two points may be the hot (ungrounded) conductor
and the neutral (grounded) conductor of a 125-volt
circuit. In this case, the voltmeter is said to be
connected across the line.
LINE VOLTAGE INDICATORS
The line voltage indicator (fig. 7-20) is much more
durable than most voltmeters for rough construction
work. Its durability is mainly due to its simple design
and construction. It has no delicate meter movement
inside the case as do the analog meters previously
mentioned. The two test leads are permanently
connected to a solenoid coil inside the molded case.
Do not use the line voltage indicator on
voltages exceeding the capabilities of the
An indicator, attached to the solenoid core, moves
along a marked scale when the leads are connected
across a voltage source. The movement of the core is
Figure 7-20.Line voltage indicator.
resisted by a spring. The indicator comes to rest at a
point along the scale that is determined by both the
strength of the magnetic field around the solenoid and
the pressure of the opposing spring. The strength of the
magnetic field is in proportion to the amount of voltage
In the center of the tester is a neon lamp indicator.
The lamp is used to indicate whether the circuit being
tested is ac or dc.
When the tester is operated on ac, it produces light
during a portion of each half-cycle, and both lamp
electrodes are alternately surrounded with a glow. The
eye cannot follow the rapidly changing alternations so
that both electrodes appear to be continually glowing
from ac current. Two other indications of ac voltage are
an audible hum and a noticeable vibration that can be
felt when the instrument is hand-held.
When the tester is operated on dc, light is produced
continuously, but only the negative electrode glows;
therefore, the tester will indicate polarity on dc circuits.
Both the test probes and the glow lamp enclosure are
colored red and black. If, while you are testing a dc
circuit, the electrode of the glow lamp on the side
colored black is glowing, this glow indicates the black