Voltmeter is an electrical device which is used to measure the potential difference between two points.

It is the modified form of galvanometer. It is a high resistance galvanometer. It is always connected in parallel with the circuit component. An ideal voltmeter has infinite resistance.

Conversion of galvanometer into voltmeter

High Resistance

A galvanometer can be converted into voltmeter by connecting a very high resistance Rn in series with meter- movement (coil) of galvanometer.

Range of voltmeter

Let, the range of the voltmeter = V (volts)

Calculation of High resistance (R)

The value of high resistance Rh should be such that galvanometer gives full scale deflection when it is connected across V volt.

Let,

The resistance of meter-movement = Rg

Full scale deflection current = Ig

Since Rg and Rh are in series, so

The combined resistance of meter-movement and Rh = Rg + Rh

Applying Ohm's law, we have

V = Ig (Rg + Rh)

V = IgRg + IgRh

or IgRh = V - Ig Rg

or Rh = V / Ig -Rg

This equation gives the value of high resistance Rh which is used to convert the galvanometer into voltmeter. The scale is calibrated from 0 to V to measure the voltage. In order to increase the range of voltmeter, Rh should be increased.

Note

The resistance of voltmeter should be greater than the circuit resistance otherwise it will change the value of potential difference which is to be measured.