Question

Shown in the figure below is a meter-bridge set up with null deflection in the galvanometer. The value of the unknown resistor $R$ is

• A. $220\Omega$
• B. $110\Omega$
• C. $55\Omega$
• D. $13.75\Omega$

Answer 1

The correct option is A $220\Omega$

Equivalent circuit of meter- bridge will be as shown below.


Applying condition for balanced Wheatstone bridge for the given situation,

$\frac{55}{20}=\frac{R}{100-20}$

$\therefore R=220\Omega$

Hence, option $\left(a\right)$ is correct.

If deflection in galvanometer is zero, then the condition for balanced Wheatstone bridge can be applied. $RQ=PX$ $X=R\times \frac{Q}{P}\Rightarrow X=\frac{100-l}{l}R$