Voltmeter

In an electric circuit, a voltmeter connected in parallel with the circuit element measures the potential difference between the two points of the circuit element.

An ideal voltmeter should have infinite resistance so that it does not draw current from the circuit. However, real voltmeters have finite resistance.

It is desirable to have a large value for resistance so that connecting the voltmeter to a circuit does not appreciably alter the measured potential difference.

Any galvanometer can be converted into a voltmeter by adding a current-limiting resistor in series.

The potential difference required for full-scale deflection for just the galvanometer is in millivolts.

The galvanometer's range can be increased by connecting a high-value resistor in series with the coil.

This configuration distributes the total potential difference between the two components in series. As a result, only a fraction of the total potential difference appears across the coil.

From this expression, the value of the series resistor required for designing a voltmeter with full scale reading can be determined.