Potentiometer

When a voltmeter is connected across the battery, it measures only the terminal voltage.

For accurate emf measurements, the battery's internal resistance should be known, or the current delivered by the battery should be zero. However, voltmeters draw small current to operate.

So null measurement devices like potentiometers are more accurate than voltmeters, as it balances the voltage and allows no current flow through the measuring device.

It consists of a voltage source connected to a long uniform resistive wire.

The voltage source maintains a constant current flow and a uniform potential gradient across the wire.

The unknown emf source is connected with a galvanometer and a jockey. The contact point is adjusted such that the emf equals the potential drop across the segment of wire and the galvanometer shows zero deflection.

A standard emf source replaces the unknown source, and the contact point is again adjusted to get zero deflection.

The ratio of the two relations gives the expression of unknown emf in terms of resistances, which is proportional to the obtained balanced lengths.