(a) Principle of potentiometer: When a steady current flows through it, the potential difference across a uniform wire is directly proportional to the length of the part across which the potential is measured.
(b) The smallest potential difference that can be measured by potentiometer is called sensitivity of potentiometer. It can be increased by (i) increasing the length of the wire (ii) decreasing the current in the wire using a rheostat
(c) Potentiometer is preferred over voltmeter to measure emf of a cell because it measures true emf of the cell. It uses null method, so no current is drawn by the galvanometer from the cell in balanced condition of potentiometer.
On the other hand, a voltmeter measures the voltage across the terminals of a cell when the cell is in closed circuit, that is, when current is flowing through the cell. This voltage is not equal to the emf of the cell. In fact, it is equal to the emf minus the potential drop across the internal resistance of the cell.