The same amount of current must always be used to deflect the pointer to a certain position on the
scale (midscale position, for example), regardless of the multiplication factor being used. Since the
multiplier resistors are of different values, you must always "zero" the meter for each multiplication scale
selected. When selecting a range on the ohmmeter, select the one that will result in the pointer coming to
rest as close to the midpoint of the scale as possible. This will enable you to read the resistance more
accurately because scale readings are more easily interpreted at or near midpoint.
An ordinary ohmmeter cannot be used for measuring multimillion ohm values of resistances, such as
those in conductor insulation. To test for such insulation breakdown, you need to use a much higher
potential than that supplied by the battery of an ohmmeter. This potential is placed between the conductor
and the outside of the insulation. A megger (megohmmeter) is used for these tests. The megger, shown in
figure 3-15, is a portable instrument consisting of two main elements: (1) a hand-driven dc generator,
which supplies the necessary voltage for making the measurement, and (2) the instrument portion, which
indicates the value of the resistance you are measuring. The instrument portion is of the opposed-coil
type, as shown in view A. Coils a and b are mounted on movable member c. A fixed angular relationship
exists between coils, and they are free to turn as a unit in a magnetic field. Coil b tends to move the
pointer counterclockwise, and coil a tends to move it clockwise.
Figure 3-15.Megger internal circuit and external view.
Coil a is connected in series with R3 and unknown resistance Rx. The combination of coil a, R3, and
Rx forms a direct series path between the + and - brushes of the dc generator. Coil b is connected in series
with R2, and this combination is also connected across the generator. Notice that the movable member
(pointer) of the instrument portion of the megger has no restoring springs. Therefore, when the generator
is not being operated, the pointer will float freely and may come to rest at any position on the scale.