Definition
Potentiometer is an accurate device that is used to measure or compare the potentials without drawing any current from the circuit.
Principle
When steady current flows through the wire of uniform cross-section area then the potential difference across any length of wire is directly proportional to its length i.e.,
, when I = constant
Construction
A potentiometer consists of a resistor in form of a wire of length L on which a terminal c can slide. Let, R is the resistance of a wire. As the sliding contact moves from A to B, the resistance between A and C changes from O to R.
Potentiometer as Potential divider
Let suppose E be the emf of the cell. The current flowing through it is
If R be the resistance between A and C then the potential drop between these points will be
Putting the value of I from the equation, we get
As E and R are constant for a given circuit, therefore the potential drop can be varied by changing the value of r. This arrangement is known as a potential divider.
Measurement of emf
A potentiometer can be used for measuring the unknown emf of a cell. It consists of a resistance wire R of a uniform cross-sectional area. let say E x is the unknown emf of the cell. It should be kept in mind that the positive terminal of the cell and that of the potential divider are connected to the same point A.
When the terminal C and negative terminal of E x are at the same potential, then no current will flow through the galvanometer. Under this condition, the emf E x of the cell is equal to the potential difference between A and C i.e., 
In the case of a wire of uniform cross-sectional area, the resistance is directly proportional to length i.e., Now the equation will become
Where l is the length of wire between A and C and L is the total length of wire.
Comparing emf of two cells
The method which is used to measure the emf of a cell can also be used to compare E1 and E2 of two cells. Let l1 and l2 are the balancing lengths for EMFs of two cells E1 and E2 respectively. Li and l2 are calculated separately for each cell.
Dividing both: 
The ratio of the EMFs is equal to the ratio of their balancing length.
Uses of Potentiometer
It can be used
- For determination of emf of the cell.
- For comparing EMFs of two cells.
- As a continuously potential divider.
- For measuring the internal resistance of the cell.