A variation in the additional circuitry permits the use of the iron-vane
movement in the design of a voltmeter. The 1 mA, 43 Ω movement can
also be rated as a 43 mV (1 mA x 43 Ω), 43 Ω movement, indicating
that the maximum voltage that the movement can measure independently
is 43 mV. The millivolt rating is sometimes referred to as the
voltage sensitivity (VS). The basic construction of the voltmeter is shown
in
Fig. 1.
Fig. 1: Basic voltmeter.
Fig. 2: Multirange voltmeter.
The $R_{series}$ is adjusted to limit the current through the movement to
1 mA when the maximum voltage is applied across the voltmeter. A
lower voltage simply reduces the current in the circuit and thereby the
deflection of the movement.
Applying
Kirchhoff's voltage law around the closed loop of
Fig. 1,
we obtain
$$[10 V - (1 mA)(Rseries)] - 43 mV = 0$$
or
$$R_{series} = 10 V - (43 mV) 1 mA = 9957 Ω =10 kΩ$$
In general,
$$\bbox[5px,border:1px solid blue] {\color{blue}{R_{series} = {V_{max} - V_{VS} \over I_{CS}}}} \tag{1}$$
How multirange voltmeter is designed?
One method of constructing a multirange voltmeter is shown in
Fig. 2. If the rotary switch is at 10 V,
$$R_{series} = 10 kΩ$$
at 50 V,
$$R_{series} = 40 kΩ + 10 kΩ = 50 kΩ$$
and at 100 V,
$$R_{series} = 50 kΩ + 40 kΩ + 10 kΩ = 100 kΩ$$