Wheat stone’ bridge works on kirchoff’ law. It is used to measure medium range resistances. P and Q are fixed standard resistances. * is standard variable resistances. X is the unknown resistance. G is galvanometer, kG is galvanometer switch and kB is battery switch. No current will flow through galvanometer if the potentials across its terminals are equal. So there will be no deflection of galvanometer. This condition is called the balanced condition.
This is achieved by varying * and also by varying P/Q ratio.
At balanced condition VAB = VAC And VBD = VCD
I1 P = I2 X & I1Q = I2S
= P/Q = X/*
Unknown resistance (X) = P/Q Ω
