Conversion of a Galvanometer into Voltmeter

Conversion of a Galvanometer into Voltmeter

A voltmeter is an instrument used to find the potential difference across any two points in a circuit. A galvanometer can directly measure small potential differences only. To measure high potential differences, we have to do some modifications in the galvanometer. What we get after modifications is known as voltmeter.

Suppose we want to make a voltmeter that can measure the potential difference up to V, the range of the voltmeter is 0 – V. For this, a suitable high resistance is connected in series with the galvanometer such that when a potential difference of V is applied, only a current IG passes through the galvanometer as shown in the figure. We can write:

Conversion of a Galvanometer into Voltmeter

V = IG (G + R) \(\Rightarrow R=\frac{V}{{{I}_{G}}}-G\),

The above equation gives the value of R in terms of V.

Example: a galvanometer has a resistance of  and it’s full – scale deflection current is 50μA. What resistance should be added to it so that it can have a range of 0 – 5 V?

Solution: Given,

IG = 50μA

Maximum voltage to be measured is V = 5V. The galvanometer resistance G = 50 Ω now:

\(R=\frac{V}{{{I}_{G}}}-G=\frac{5}{50\times {{10}^{-6}}}-50\approx 100K\Omega \),

∴ R = 100 KΩ.