Answer the following:
(a) Write the principle of working of a metre bridge.
(b) In a metre bridge, the balance point is found at a distance `I_(1)` with resistances R and S as shown in the figure.

An unknown resistance `X` is now connected in parallel to the resistance `S` and the balance point is found at a distance `I_(2)` , Obtain a formula for `X` in terms of `I_(1), I_(2)` and `S` .