State the principle of a potentiometer. With a labelled circuit diagram, explain how it is used to compare the EMFs of two cells. Why is it preferred to a voltmeter?

1. 1 · State the principle. For a steady current in a wire of uniform cross-section, the potential drop across any length is proportional to that length: V = kl.
2. 2 · Draw the circuit. [Wire AB with a driver cell, key K and rheostat; cells E₁ and E₂ with their + terminals at A, through a two-way key, galvanometer G and jockey J]
3. 3 · Balance the first cell. Connect E₁ and slide the jockey until G shows no deflection, at length l₁: E₁ = kl₁.
4. 4 · Balance the second cell. With the same current in AB, connect E₂ and find l₂: E₂ = kl₂.
5. 5 · Result. E₁/E₂ = l₁/l₂.
6. 6 · Why better than a voltmeter. At balance no current is drawn from the cell, so it measures the true EMF; the galvanometer only shows zero, so it need not be calibrated.
7. 7 · Close in one line. So a potentiometer compares EMFs by comparing lengths, without drawing current.

Scoring tip: Say that the current in the main circuit stays the same while you balance both cells. Without it, the ratio l₁/l₂ means nothing.