Principle of potentiometer. The fall of potential along any length of the wire is directly proportional to that length. When a constant current flows through a wire of uniform cross-section and composition.V ∝ lComparison of emfs of two primary cells: The circuit diagram is shown in the figure.When the key K is closed, a constant current flows the potentiometer wire. By closing key K1, the cell E1 is included in the circuit. The jockey is adjusted till galvanometer shows no deflection. Suppose AJ1 = l1 is the balancing length for cell E1. ThenE1 = kl1where k is the potential gradient. Now the null point is obtained for cell E2 by closing key K2. Let AJ2 = l2 be the balancing length in this case. Then E2 = kl2                      by increasing the length of a potentiometer s wire the sensitivity of a potentiometer can be increased.