VOLTMETERS
Test Instruments & Measuring Devices
V = ImRs + ImRm
ImRs
= V - ImRm
Rs =
(14-1)
V
Im
Rm
where
V = voltage range desired
Im = meter current
Rm = meter resistance
Rs = multiplier resistance or series resistance
Example:
A 2 mA meter movement with internal resistance of 25 ohms is to be constructed
as a voltmeter.
What value must the series resistance be to measure full scale voltage of 100
volts?
Solution:
Rs
V
Im
Rm
Since Rm is negligibly low, then:
Rs
V
Im
100
2x103
Rs
50kW
When a voltmeter is connected in a circuit, the voltmeter will draw current from that circuit.
This current causes a voltage drop across the resistance of the meter, which is subtracted from
the voltage being measured by the meter. This reduction in voltage is known as the loading
effect and can have a serious effect on measurement accuracy, especially for low current circuits.
ES-14
Page 6
Rev. 0
