A battery of stable emf E is used to set up a potential gradient V/L, along the potentiometer wire, where V = potential difference across length L of the wire. The positive terminal of the cell 1 is connected to the higher potential terminal A of the potentiometer; the negative terminal is connected to the galvanometer G through the reversing key. The other terminal of the galvanometer is connected to a pencil jockey. The cell 2 is connected across the remaining two opposite terminals of the reversing key. The other terminal of the galvanometer is connected to a pencil jockey. The emf E1 should be greater than the emf E2 ; this can be adjusted by trial and error.
Two plugs are inserted in the reversing key in positions 1 – 1. Here, the two cells assist each other so that the net emf is E1 + E2 . The jockey is tapped along the wire to locate the null point D. If the null point is a distance l1 from A,
E1 + E2 = l1 (V/L)
For the same potential gradient (without changing the rheostat setting), the plugs are now inserted into position 2 – 2. (instead of 1 – 1). The emf E2 then opposes E1 and the net emf is E1 – E2 . The new null point D’ is, say, a distance l2 from A and
E1 – E2 = l2 (V/L)
Here, the emf E should be greater than E1 + E2 . The experiment is repeated for different potential gradients using the rheostat.
[Note : This method is used when E1 » E2 , so that E1 + E2 and E1 – E2 have comparable magnitudes. Then, the errors of measurement of l1 and l2 will also be of comparable magnitudes.]
