Voltmeter Design

Voltmeters, as well as ammeters and ohmmeters, are designed with the use of a sensitive current detector such as a galvanometer.

A voltmeter is placed in parallel with a circuit element to measure the voltage drop across it and must be designed to draw very little current from the circuit so that it does not appreciably change the circuit it is measuring. To accomplish this, a large resistor is placed in series with the galvanometer. Its value is chosen so that the design voltage placed across the meter will cause the meter to deflect to its full-scale reading. A galvanometer full-scale current is very small: on the order of milliamperes.

If a galvanometer with RG = Ω , IG = mA

is used to design a voltmeter for a full scale voltage V = volts, the required series resistor is given by

= Ω