Voltmeter

A voltmeter is used to measure the potential difference between two points in a circuit. You can convert a galvanometer into a voltmeter by connecting a high resistance in series with the galvanometer coil.

The value of the resistance depends on the range of voltmeter. A high resistance, say R is connected in series with the galvanometer coil. If the potential difference across AB is V volt, then total resistance of the voltmeter will be G + R. From Ohm’s law,

I_g(G + R) = V

G + R = v/I_g

R = V/I_g - G

If a resistance R is connected in series with the coil of the galvanometer, it works as a voltmeter of range 0-V volts.

The same scale of the galvanometer which was recording the maximum potential I_g×G before conversion will record the potential V after conversion into voltmeter. The scale can be calibrated accordingly. The resistance of the voltmeter is higher than the resistance of galvanometer. Effective resistance of the voltmeter, is given by

R_V = R + G

The resistance of an ideal voltmeter is infinite. It is connected in parallel to the points across which potential drop is to be measured in a circuit. It will not draw any current. But the galvanometer coil deflects.