(a) Write the principle of working of a metre bridge.
(b) In a metre bridge, the balance point is found at a distance ${\mathit{l}}_1$ with resistances $\mathit{R}$ and $\mathit{S}$ as shown in the figure.
An unknown resistance $\mathit{X}$ is now connected in parallel to the resistance $\mathit{S}$ and the balance point is found at a distance ${\mathit{l}}_2$. Obtain a formula for $\mathit{X}$ in terms of ${\mathit{l}}_1,{\mathit{l}}_2$ and $\mathit{S}$.