Question

The $\text{AC}$ voltage across a resistance can be measured using

• A. a potentiometer
• B. a hot-wire voltmeter
• C. a moving-coil galvanometer
• D. a moving-magnet galvanometer

Answer 1

The correct option is B a hot-wire voltmeter

Ordinary $\text{DC}$ voltmeter cannot measure alternating voltages. When used in $\text{AC}$ circuits, these instruments record zero reading because the average value of alternating voltage over a full cycle is zero.

Alternating voltage across the resistance can be measured by using an $\text{AC}$ voltmeter. Such $\text{AC}$ voltmeter is known as a hot wire voltmeter which measures only virtual values(r.m.s values) of the alternating voltages.

The hot wire voltmeter works on the principle of heating effect of current.

Moving coil or magnet galvanometer can measure only $\text{DC}$ values. When $\text{AC}$ passes through a galvanometer it will not show any deflection, because the impulses to the moving parts of the galvanometer are equal and opposite.

Hence, $\left(B\right)$ is the only correct answer.