The metre bridge, consists of a one metre long wire of uniform cross sectional area, fixed on a wooden block. A scale is attached to the block. Two gaps are formed on it by using thick metal strips in order to make the Wheat stone’s bridge. The metre bridge is operates under Wheat stone’s principle. Here, four resistors P, Q, R, and S are connected to form the network ABCD. In the balancing condition, there is no deflection on the galvanometer. Then, P/q = R/S.
This video explains how to verify the laws of resistances in series and parallel.