CBSE Class 12 · 2015 · Delhi · Current ElectricityShort Answer

Label the four arms P, Q, R, S with the galvanometer G across the bridge. Applying the loop rule to the two upper and lower loops with the galvanometer current $I_g=0$ at balance: the currents through P and Q are equal (say $I_1$) and those through R and S are equal (say $I_2$). The loop containing P and R gives $I_{1}P=I_{2}R$, and the loop containing Q and S gives $I_{1}Q=I_{2}S$. Dividing,