Question

Explain how do you convert a moving coil galvanometer into a voltmeter?

Answer 1

Galvanometer is a very sensitive instrument, therefore it can not measure high potential difference. In order to convert a Galvanometer into voltmeter, a very high resistance known as "series resistance" is connected in series with the galvanometer.
Calculation of value of required resistance:
Let resistance of galvanometer = $R_g$ and resistance $R_x$ (high) is connected in series to it. Then combined resistance $=(R_g+R_x)$.
Let the potential between the points to be measured be $V$ and if galvanometer gives full-scale deflection, when current $I_g$ passes through it. Then,

$V=I_g(R_g+R_x)$

$V=I_g{R}_g+I_g{R}_x$

$V–I_g{R}_g=I_g{R}_x$

$R_x=\frac{V}{I_g}-R_g$