The Voltmeter

The voltmeter uses the same type of meter movement as the ammeter but employs a different circuit external to the meter movement.

As shown before, the voltage drop across the meter coil is a function of current and the coil resistance. In another example, 50μA × 1000Ω = 50mV. In order for the meter to be used to measure voltages greater than 50mV, there must be added a series resistance to drop any excess voltage greater than that which the meter movement requires for a full scale deflection. The case of the voltmeter, this resistance is called multiplier resistance and will be designated as RM. Figure 145 illustrates a basic voltmeter. This voltmeter only has one multiplier resistor for use in one range. In this example, the full scale reading will be 1 volt. RM is determined in the follow way:

The meter movement drops 50mV at a full scale deflection of 50μA. The multiplying resistor RM must drop the remaining voltage of 1V − 50mV = 950 mV. Since RM is in series with the movement, it also carries 50μA at full scale.