Page 832 - Basic Electrical Engineering

DC voltmeter: A dc milliammeter can be converted as a dc voltmeter by
connecting a high resistance in series with the instrument. Such a high
resistance is called multiplier. A dc voltmeter measures the potential

difference between two points, and hence is connected across the terminals.
The high resistance connected in series with the instrument limits the current

flowing through the coil of the instrument. The value of the multiplier is
calculated as shown in Fig. 11.18.

Figure 11.18

Let R be the series resistance of the multiplier as shown in Fig. 11.18.
I is the full-scale deflection current of the instrument.
m
R is the resistance of the moving coil.
m
V is the desired full-scale voltage range of the instrument being used as a
voltmeter.

From the circuit of Fig. 11.18

V = I (R + R)
m
m

or,

For example, let R = 50 Ω, I = 1 mA and the range of the voltmeter be 0–
m
m
100 V.