The Potentiometer

A potentiometer measures or compares e.m.f. by finding a position where a detector reads zero.
At that balance point, the test cell supplies no current. Its internal resistance therefore causes no lost volts.

Two circuits meet at the jockey

A hand-drawn potentiometer experiment with a driver cell across a long uniform wire, and a test cell connected through a centre-zero galvanometer to a sliding jockey — The upper driver circuit establishes a steady potential gradient along the wire. The lower test circuit carries no current when the jockey reaches the balance point.

Slide Z until the galvanometer reads exactly zero. At that point the test cell drives no current at all, so its full e.m.f. is being matched by the wire's p.d. between X and Z.

The driver circuit maintains a steady current through the uniform wire, producing a constant potential gradient.
The test circuit contains the cell, galvanometer and movable contact. Move the contact until the galvanometer reads zero.
If the test cell is reversed, its e.m.f. reinforces rather than opposes the wire p.d., so a balance cannot be found.

Why the null method is accurate

Definition

1 mark

What is meant by null method?

Model answer: A measurement method in which a detector reading is adjusted to zero.

At null, the p.d. along the balance length equals the test e.m.f. No current leaves the test cell, so the measured value is not reduced by its internal resistance.

Your turn9702/23/M/J/25 Q7(b)(i)

State what is meant by the null position of a potentiometer. [1] (9702/23/M/J/25 Q7)

Show worked answer

It is the contact position at which the galvanometer reads zero, so no current flows in the test-cell branch.

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