Question

The resistance of a voltmeter is $180\Omega$. When there is $5V$ potential difference across its ends, it gives full scale deflection. What resistance should be connected in series to obtain a full scale deflection at $25V$?

Answer 1

$5=I_g\times 180$ (1)

$25=I_g(180+R)$ (2)

From (1) and (2), we get

$\frac{1}{5}=\frac{180}{180+R}$

$180+R=900$

$\therefore R=720\Omega$