State the advantages of a potentiometer over a voltmeter.

Answer:

1. A potentiometer uses a null method: at balance it draws no current from the cell, so it measures the true e.m.f. A voltmeter always draws some current, so it measures only the terminal p.d., which is less than the e.m.f.
2. A potentiometer is more accurate and sensitive; its sensitivity can be increased by increasing the length of the wire (reducing the potential gradient).
3. It can measure very small potential differences accurately.
4. It can also be used to measure internal resistance of a cell, compare e.m.f.s and resistances, which an ordinary voltmeter cannot do.

(A voltmeter is, however, more convenient and portable and gives a direct reading.)