Conversion of galvanometer into a voltmeter:
Voltmeter is an instrument used to measure potential difference between the two ends of a current carrying conductor.
A galvanometer can be converted into a voltmeter by connecting a high resistance in series with it. The scale is calibrated in volt. The value of the resistance connected in series decides the range of the voltmeter.
Galvanometer resistance = G
The current required to produce full scale deflection in the galvanometer = Ig
Range of voltmeter = V
Resistance to be connected in series = R
Since R is connected in series with the galvanometer, the current through the galvanometer,
Ig = V/(R+G)
so, R = V/Ig – G
From the equation the resistance to be connected in series with
the galvanometer is calculated.
The effective resistance of the voltmeter is
Rv = G + R
Rv is very large, and hence a voltmeter is connected in parallel in a circuit as it draws the least current from the circuit. In other words, the resistance of the voltmeter should be very large compared to the resistance across which the voltmeter is connected to measure the potential difference. Otherwise, the voltmeter will draw a large current from the circuit and hence the current through the remaining part of the circuit decreases. In such a case the potential difference measured by the voltmeter is very much less than the actual potential difference.
The error is eliminated only when the voltmeter has a high resistance. An ideal voltmeter is one which has infinite resistance.