Question

In an ammeter, 4% of the main current is passing through the galvanometer. If shunt resistance is $5\Omega$, then resistance of galvanometer will be:

• A. $60\Omega$
• B. $240\Omega$
• C. $120\Omega$
• D. $480\Omega$

Answer 1

The correct option is C $120\Omega$

Shunt is a low resistance connected in parallel with the galvanometer or ammeter.
If the current flowing in a circuit is strong, a galvanometer or an ammeter cannot be put directly in it because the instrument may be damaged. To overcome this difficulty, a low resistance (ie, shunt) is connected in parallel with this instrument. Then a major portion of the current passes through this low resistance (ie, shunt) and only a small portion passes through the instrument. Due to it, the galvanometer or an ammeter remains safe.
Shunt is given by
$S=\frac{I_{g}G}{I-I_g}$
$\therefore 5=\frac{\frac{4}{100}IG}{I-\frac{4}{100}I}=\frac{4G}{96}$
or $G=\frac{96\times 5}{4}=120\Omega$