probe of the tester is on the negative side of the circuit;
likewise, the opposite electrode glows when the red
probe of the tester is on the negative side of the circuit.
The neon lamp is not the only method used on line
voltage indicators to indicate dc polarity; for example,
the Wigginton voltage tester, manufactured by the
Square D Company, uses a permanent magnet mounted
on a rotating shaft The ends of the magnet are colored
red and black. The magnet is viewed from a transparent
cap located on top of the tester. When the red portion of
the magnet is up, the red test prod is positive. When the
black portion of the magnet is up, the black prod is
positive. Neither type of line voltage indicator vibrates
when measuring dc.
Be certain to read and understand the instructions
for the particular instrument you use. As you can see
from this example about polarity indicators, because of
variations in similar instruments, you could easily
misunderstand an indication from one instrument when
thinking of the instructions for another.
The line voltage indicator is not used to determine
the exact amount of circuit voltage. That presents no
problem for most of the work done by Construction
Electricians. As you become proficient in the use of the
solenoid type of voltage indicator, you can tell
approximately what the voltage is by the location of the
indicator within a voltage range on the scale.
The resistance of a component or circuit, in ohms,
can be determined by using Ohms law. With the
instruments we just discussed, you can find circuit
current and voltage. From electrical theory you already
know that voltage divided by amperage equals
resistance. But the fastest method of determining
resistance is by taking a resistance reading directly from
The simplest type of ohmmeter consists of a
housing that includes a milliammeter, a battery, and a
resistor connected in series, as shown in figure 7-21.
The ohmmeter is designed so that the resistor R1 limits
the current though the milliammeter to a value that
results in a full-scale deflection of the meter needle.
The scale (fig. 7-22) is calibrated in ohms. By using
several resistors, more than one battery, and a selector
switch (to select one of the several resistors and
batteries), you can make the ohmmeter include more
than one resistance range.
Figure 7-21.A simple series ohmmeter circuit.
You may use a variable resistor in the meter circuit
(R2 in fig. 7-21) to compensate for variations in battery
voltage. Before using an ohmmeter for a precise
resistance measurement, short the leads together and set
the needle to zero by rotating the zero ohms (variable
resistor) knob. The result is a full-scale reading at zero
Be certain not to place the ohmmeter leads
across an energized circuit or a charged
capacitor. Ignoring this rule will likely result in
damage to the test equipment. Always turn off
the power on a circuit to be tested before
making continuity or resistance tests. Before
you test with an ohmmeter, bleed any
capacitors that are included in the circuits under
test. Use extreme care in testing solid-state
components and equipment with an ohmmeter.
The voltage from the internal batteries of the
ohmmeter will severely damage many solid-
state components. Always turn an ohmmeter
off after you have completed your test to
lengthen the life of the batteries.
Figure 7-22.Typical scale of a series type of ohmmeter.