State the principle of a Wheatstone bridge.

(N/A) The principle of a Wheatstone bridge states that if four resistors $P, Q, R,$ and $S$ are arranged in a bridge configuration such that no current flows through the galvanometer $(I_g = 0)$,the bridge is said to be balanced.
In this balanced condition,the ratio of the resistances in the two arms is equal,which is given by the formula: $\frac{P}{Q} = \frac{R}{S}$.
This condition is achieved when the potential difference across the galvanometer is zero,meaning the potentials at the two connected points are equal.