Question

A moving coil galvanometer of resistance $20\Omega$ gives a full scale deflection when a current of $1\text{mA}$ is passed through it. It is to be converted into an ammeter reading $20\text{A}$ on full scale. But the shunt of $0.005\Omega$ only is available. What resistance should be connnected in series with the galvanometer coil?

• A. $39.995\Omega$
• B. $60.995\Omega$
• C. $79.995\Omega$
• D. $80.995\Omega$

Answer 1

The correct option is C $79.995\Omega$

$S=\frac{i_g}{i-i_g}(G+R)$

$R=\left[\frac{i-i_g}{i_g}\right]S-G$

$=\frac{20-{10}^{-3}}{{10}^{-3}}\times 0.005-20$

$=79.995\Omega$