A battery of emf E is connected the end terminal A and B of potentiometer with rheostat Rh, resistance box and key K in series. This is called auxiliary circuit.
The ends of the R1 are connected to terminal A and jockey J through galvanometer. The resistance R1 is connected to cell E1 and key K1 in series.
Working and Theory: Close key K and adjust the resistance from resistance box so that the potential across the potentiometer wire is grated then which is to be measured.
Close key K1. The current flows through R1. A potential difference is developed across R1. Adjust the position of jockey on potentiometer wire such that galvanometer shows no deflection.
Let this position is J. Let AJ length is l of potentiometer wire. Galvanometer shows no deflection because potential difference is same if K is the potential gradient of potentiometer wire, then potential difference across R1 that is
Potentiometer does not draw any current from the cell whose emf is to be measured, whereas a voltmeter always draws some current.