The AC voltage across a resistance can be measured using a potentiometer, moving coil galvanometer, hot wire voltmeter, or moving magnet galvanometer.

Ordinary DC voltmeters cannot measure alternating voltages. When used in AC circuits, these instruments record a zero reading because the average value of the alternating voltage over a full cycle is zero. Alternating voltage across the resistance can be measured by using an AC voltmeter. Such an AC voltmeter is known as a hot wire voltmeter, which measures only the virtual values (r.m.s values) of the alternating voltages. Moving coil or magnet galvanometers can measure only DC values. When AC passes through a galvanometer, it will not show any deflection because the impulses to the moving parts of the galvanometer are equal and opposite. To measure AC, the heating effect of the current is used because the heating effect does not depend on the direction of flow of current. The hot wire voltmeter works on the principle of the heating effect of current.