(i) Principle of Potentiometer: When a constant current flows through a wire of uniform area of cross-section, the potential drop across any length of the wire is directly proportional to the length.
Circuit Diagram. It consists of a long resistance wire AB of uniform cross-section. Its one end A is connected to the positive terminal of battery B1 whose negative terminal is connected to the other end B of the wire through key K and a rheostat (Rh). The battery B1 connected in circuit is called the driver battery and this circuit is called the primary circuit. By the help of this circuit a definite potential difference is applied across the wire AB; the potential falls continuously along the wire from A to B. The fall of potential per unit length of wire is called the potential gradient. It is denoted by ‘k’. A cell is connected such that its positive terminal is connected to end A and the negative terminal to a jockey J through the galvanometer G. This circuit is called the secondary circuit.
In primary circuit the rheostat (Rh) is so adjusted that the deflection in galvanometer is on one side when jockey is touched on wire at point A and on the other side when jockey is touched on wire at point B.
The jockey is moved and touched to the potentiometer wire and the position is found where galvanometer gives no deflection. Such a point P is called null deflection point.
VAB is the potential difference between points A and B and L metre be the length of wire, then the potential gradient
\(k=\frac{V_{AB}}{L}\)
If the length of wire AP in the null deflection position be l, then the potential difference between points A and P,
VAP = KI
∴ The emf of cell, ε = VAP = kl
In this way the emf of a cell may be determined by a potentiometer.
