potentiometer An instrument for measuring and comparing small potential difference. It consists of a long wire of uniform cross section (see fig. p11). A constant emf source connected across two ends of the wire produces a uniform potential gradient along its length. Therefore the voltage at any point along its length is proportional to the distance from the end ‘A’. A sliding contact is connected through a key and galvanometer to one output terminal. The end ‘A’ is connected to the other output terminal.

The potentiometer is used to compare emf’s of two cells. A known emf source, of emf e, is connected to the output terminals. The null deflection in the galvanometer is obtained by adjusting the variable resistance R, and the position of the sliding contact. Let this length be s from end ‘A’. At this position there is no current through the cell. Now the known source is replaced with the unknown source, of emf e¢ , and again the balancing length required is found. Let that be s¢ . Then the ratio of the emf’s of the two sources is,

e/e¢ = l/l¢

This instrument gives an accurate value of the emf of a cell, as it measures the emf without drawing any current from the cell.