The metre bridge is operates under Wheatstone’s principle. Here, four resistors P, Q, R, and S are connected to form the network ABCD. The terminals A and C are connected to a battery, and the terminals C and D are connected to a galvanometer through keys K1 and K2 respectively. In the balancing condition, there is no deflection on the galvanometer. Then, P/Q = R/S.
This video explains how to find the resistance of a given wire using a metre bridge and hence determine the specific resistance of its materials.