Determination of Potential Difference using Potentiometer

A battery of emf E is connected the end terminal A and B of potentiometer with rheostat R_h, 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.