State the working principle of a potentiometer

(a) State the working principle of a potentiometer. With the help of the circuit diagram, explain how a potentiometer is used to compare the emfs of two primary cells. Obtain the required expression used for comparing the emfs.
(b) Write two possible causes for one sided deflection in a potentiometer experiment.

(a) Principle : When a constant current flows through a wire of uniform area of cross-section then potential difference between two points on the wire is directly proportional to length of this section of wire.

(b) (i) When the driver cell / source cell has emf less than the emf of the cells to be compared.
(ii) When the positive end of the potentiometer wire is connected to negative terminal of the cell whose emf is to be compared /determined.