Components of an emf potentiometer

Driver Cell: A cell with a known and stable emf that provides the main voltage supply to the circuit.
Test Cell: The cell whose emf ($E_{x}$) is to be measured. The cell must have a lower voltage than the driver cell.
Potentiometer Wire: A uniform resistance wire, stretched between two points (A and B). This wire acts as a uniform potential divider, meaning the voltage drops steadily along its length.
Jockey: A metal contact with a sharp edge that can be moved along the potentiometer wire to make contact at different points.
Sensitive Galvanometer: A device used to detect very small currents. It is used to indicate when the potentiometer circuit is balanced.

Using a potentiometer

The driver cell (with emf E)is connected across A to B, creating a steady voltage drop along the wire.
When the jockey is close to point A, the galvanometer will deflect in one direction. When the jockey is close to point B, the galvanometer will deflect in the other.
A point C will be found on the wire where touching the jockey causes zero deflection on the galvanometer. This is the balance point.
At this balance point, the potential difference across the portion of the wire between point A and point Y is exactly equal to the emf of the test cell ($E_{x}$).
If the length of the wire from A to the balance point C is AC, the emf of the test cell ($E_{x}$) can be calculated as: $$E_x = \left(\frac{AC}{AB}\right) \times E$$