Question

How do you convert a moving coil galvanometer into a voltmeter?

Answer 1

Galvanometer is a very sensitive instrument therefore it can’t measure heavy currents. In order to convert a Galvanometer into an Ammeter, a very low resistance known as "shunt" resistance is connected in parallel to Galvanometer. Value of shunt is so adjusted that most of the current passes through the shunt. In this way a Galvanometer is converted into Ammeter and can measure heavy currents without fully deflected.

To get the value of shunt resistance, let resistance of galvanometer $=R_g$ and it gives full-scale deflection when current $I_g$ is passed through it. Then,

$V_g=I_g{R}_g$

Let a shunt of resistance ($R_s$) is connected in parallel to galvanometer. If total current through the circuit is I then current through shunt:

$I_s=I-I_g$

and potential difference across the shunt:

$V_s=I_s{R}_s$

$V_s=(I-I_g)R_s$

$V_s=V_g$

$(I-I_g)R_s=I_g{R}_g$

$R_s=\frac{I_g{R}_g}{I-I_g}$