Which of the following instruments can be used to measure current in an a.c circuit?

Analyze the question You're asked which instrument can measure current in an a.c. circuit. The key is knowing how each instrument responds to a current that keeps changing direction.

Concept

• A.c. reverses direction many times a second (50 Hz in Nigeria).
• Instruments whose deflection depends on the direction of the current (a permanent magnet interacting with a coil) cannot follow a.c. The coil gets pushed one way and then the other, so the pointer just vibrates or reads zero (the average of a sinusoidal current is zero).
• Instruments whose deflection depends on the heating effect (I²R) work for both a.c. and d.c., because heating does not depend on current direction.

Checking each option

• A. Hot wire ammeter: The current heats a thin wire, which expands and moves the pointer. Heating depends on I², not direction, so it works for a.c. and d.c. ✔
• B. Moving coil ammeter: It uses a coil in a permanent magnet's field. The torque reverses with the current, so it only works for d.c. ✘
• C. Tangent galvanometer: It depends on the current's magnetic field against the Earth's field. The reversing field gives no steady deflection, so it is for d.c. ✘
• D. Moving coil galvanometer: It works on the same principle as B, so it is for d.c. only. ✘

Answer: A. Hot wire ammeter

Tip: Remember "Hot wire = heating = both a.c. and d.c." Any instrument with a permanent magnet and a coil (moving coil) is d.c. only, unless a rectifier is added. A moving-iron instrument also works on a.c.