Sensitivity of a Wheatstone bridge = smallest change in resistance $\Delta R$ detectable by the galvanometer.
It depends on:
1. Sensitivity of the galvanometer.
2. EMF of the battery $E$.
3. Values of the four resistances — most sensitive when $P$, $Q$, $R$, $S$ are of the same order.
Balance condition: $P/Q = R/S$