The potentiometer

Last time

Before we start: last time, in Potential divider design, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

A voltmeter that takes no current

A voltmeter always draws a little current from the circuit, so it always disturbs the reading a little. The potentiometer compares an unknown e.m.f. with a known p.d. along a wire, and at balance no current flows at all. So it measures the true e.m.f. of a cell, with no lost volts.

How a potentiometer works

A driver cell sends a steady current along a uniform resistance wire, so the p.d. along the wire is proportional to length. The cell under test is connected through a galvanometer to a sliding contact P on the wire. Move P until the galvanometer reads zero. This is the null point. No current flows in the test cell. At the null point, the e.m.f. of the test cell equals the p.d. along the wire from the end to P.

The null method equation

Since the p.d. along the wire is proportional to length, E over the p.d. across the whole wire equals XP over the total length. A nichrome wire of 150 centimetres is across a 1.2 volt cell. The null point is 64 centimetres from the left end. The e.m.f. is 1.2 times 64 over 150: zero point five one volts. A null method is one in which the galvanometer reading is zero.