Page 830 - Basic Electrical Engineering

effect due to change in resistance can be avoided. The other two effects, i.e.,
reduction of spring tension causing increased reading and reduction of flux
density causing reduced reading should more or less cancel each other. The

manganin resistor which is made 20 to 30 times more than the resistance of
the moving coil is called swamping resistor.

An uncalibrated PMMC instrument is also known as galvanometer which
is generally used as a null detector in a bridge circuit. The galvanometer is

used only to detect any flow of current and not the amount of current. When a
galvanometer scale is calibrated in terms of current or voltage, it can be used

as an ammeter or a voltmeter. So, the basic moving element of a dc ammeter
or a dc voltmeter is a PMMC galvanometer. However, since the winding of
the moving coil is made up of very thin wire to keep it light, it can carry only

a very small amount of current, of the range of few milliamperes. When
calibrated in milliamperes, the instrument becomes a milliammeter. When the

galvanometer is to be converted into an ammeter of higher ranges, very-low-
value resistances are connected in parallel with the moving coil so as to

bypass the excess current (i.e., the current which is more than a few
milliamperes that the coil can carry safely). These low value resistances are

called shunts.
The value of a shunt can be calculated by equating the voltage drops across
the instrument in two parallel paths as shown in Fig. 11.17.

Figure 11.17 Basic ammeter circuit with a low-value shunt resistance

I R = R I and I = I + I s
s s
m
m
m
Therefore,