Page 833 - Basic Electrical Engineering

Then the multiplier resistance R is calculated by using relation (i) as

This shows that the value of multiplier is very high whereas a shunt
resistance used in dc ammeters is very low. The multiplier resistor is placed

inside the instrument and only the two terminals are brought out and the
range is shown on a calibrated scale. Similar to a multi-range ammeter,

voltmeters can be also made for multi-range measurement by using a number
of multipliers whose values are calculated for different voltage ranges. While
all the multiplier resistances are placed inside the instrument box, a selector

switch can be used to choose the particular range of the instrument for the
measurement of voltage accurately as shown in Fig. 11.19.

Figure 11.19 Multi-range dc voltmeter

V is for the lowest range of measurement of voltage. As the voltage range is
1
higher, higher is the value of multiplier resistance like R + R for range V ,
2
4
3
R + R + R for range V , etc.
2
3
3
4
Let us assume that the moving coil is to be converted into a voltmeter of
different ranges, viz 0–10 V, 0–50 V, 0–50 V, and 0–500 V.
For 0–10 V measurement the selector switch S has to be put at V . The
1
series multiplier resistance is R . Since, R and R are in series across 10 V
m
4
4
and for full-scale deflection I is equal to 1 mA, we can write
m