the range setting of the switch; for example, when using
the multimeter with the switch set to 300 AC VOLTS,
read from the scale that has a maximum reading of
300 ac. Simply take the reading directly from either of
the digital multimeters.
Always be alert when taking voltage or
amperage measurements if it is necessary to
move the meter. If the instrument is moved in a
way that causes tension on the test leads, one or
both leads may be pulled Tom the jack(s). The
leads will be energized just as the circuit to
which they are connected, and they can be
The positions of the jacks may differ for a particular
measurement, from one meter to another. Notice how
the jacks are labeled on the instrument you use, and
follow the instructions from the manufacturer of the
It is possible that you may never use a multimeter
for amperage measurements. Most multimeters are
designed with quite low current ranges. The clamp-on
ammeter (discussed earlier) is the most convenient
portable instrument for measuring ac amperes.
As mentioned earlier, ohmmeters have their own
voltage source. This circumstance is also true of the
ohmmeter function of multimeters. The size and
number of batteries for different instruments vary.
Usually one or more 1 1/2- to 9-volt batteries are used
for resistance measurements.
As you must set up the meter to measure voltage
accurately, so you must set it up for measuring
resistance. If you are to measure a 120-ohms resistor,
for example, set the selector switch to ohms at the
appropriate range. For the analog instruments, set the
switch to the R x 1 or x 1 as appropriate. Read the value
from the ohms scale directly. For higher values of
resistance like 1,500 ohms, for example, use the R x 100
or x 100 range. In this case, multiply the reading from
the ohms scale by 100.
For all critical resistance measurements, always
touch the leads together and set the indicator needle to
zero with the appropriate adjustment knob. Do not let
the leads touch your fingers or anything else while you
are zeroing the meter.
On multimeters, use the common (negative) and +
(positive) jacks for resistance measurements.
Be certain that there is no power on the circuit or
component you are to test when measuring resistance.
Be sure also to discharge any capacitors associated with
the circuit or component to be tested before connecting
the instrument to the circuit or component
For critical measurements, make sure that only the
circuit or component you are to test touches the leads
while you take the reading; otherwise, the reading may
be inaccurate, especially on the higher resistance
Many times you will use the ohmmeter for
continuity tests. All you will want to know is whether
the circuit is complete or not. You will not have to zero
the meter for noncritical continuity tests. You will want
to touch the leads together to see where the needle
comes to rest. If the needle stops at the same place when
you place the leads across the circuit, you will know the
path has a low resistance. In other words you will know
there is continuity through the circuit.
Construction Electricians also use other
instruments for different types of resistance
measurements. We will discuss these instruments next.
The megohmmeter is a portable instrument
consisting of an indicating ohmmeter and a source of dc
voltage. The dc source can be a hand-cranked
generator, a motor-driven generator, a battery-supplied
power pack, or rectified dc.
The megohmmeter is commonly called a "megger"
although Megger© is a registered trademark. The
megger tester shown in figure 7-25 is an example of a
dual-operated megohmmeter. having both a hand-
cranked generator and a built-in line power supply in the
Any one of the ohmmeters shown in figure 7-24
will measure several megaohms. You may wonder why
they are not called megohmmeters. What is the
difference between the megger and the typical
ohmmeter? Does not each of them have an indicator
and a dc voltage source within the instrument
enclosure? The megger is capable of applying a much
higher value of dc voltage to the circuit or component
under test than is the typical ohmmeter. Meggers that
will supply a test potential of 500 volts are common in