The circuit to determine the internal resistance of a cell using a potentiometer of is shown below: -

The potentiometer is a device used to measure the internal resistance of a cell and is used to compare the e.m.f of two cells and potential difference across a resistor. The relation between potential difference, emf, and internal resistance of a cell is given by

Here, the V=IR;

So we can write the above equation as;

The above equation reflects that the potential drop V is less than E by the amount of fall in potential inside the cell due to internal resistance.

From the above equation,

The internal resistance of the cell is given as;

where r is the internal resistance of the cell and R is the resistance for the potentiometer.