across the generator. Notice that the movable
The megger (fig. 2-6) is a portable instrument
member (pointer) of the instrument portion of the
consisting of two main elements.
megger has no restoring springs. Therefore, when
the generator is not operating, the pointer will
1. The hand-driven dc generator, which
float freely and may come to rest at any position
supplies the voltage for making the
on the scale.
measurement, and
The guard ring, shown in figure 2-6, view A,
2. the instrument portion, which shows the
shunts any leakage currents to the negative side
value of the resistance you are measuring.
of the generator. This prevents such current from
flowing through coil a and affecting the meter
The instrument portion is of the opposed-coil
reading.
type, as shown in view A. Coils a and b are on
If the test leads are open, no current will flow
movable member c. A fixed angular relationship
in coil a. However, current will flow internally
exists between coils; they are free to turn as a unit
through coil b and deflect the pointer to infinity.
in a magnetic field. Coil b moves the pointer
This reading shows a resistance too large to
counterclockwise, and coil a moves it clockwise.
measure. When you connect a resistance, such as
Coil a connects in series with R3 and unknown
RX, between the test leads, current also flows in
coil a; this moves the pointer clockwise. At the
RX forms a direct series path between the + and
same time, coil b moves the pointer counter-
brushes of the dc generator. Coil b connects
clockwise. Therefore, the moving element,
in series with R2. This combination also connects
Figure 2-6.-Megger internal circuit and external view.
2-18
