In measuring high frequency a.c, the instrument used is the

1. Analyze the question We need an ammeter that can measure high-frequency alternating current accurately.

2. Concept Different ammeters work on different principles:

• Moving coil ammeter: Uses the force on a current-carrying coil in a permanent magnet's field. The deflection direction reverses with the current, so on a.c. it just vibrates and averages to zero. It works only for d.c.
• d.c. ammeter: Same as above, so it is unsuitable for a.c.
• Moving iron ammeter: Works on both a.c. and d.c., but at high frequencies the coil's inductance and eddy-current losses make its readings inaccurate. It is good for low (mains) frequencies, such as 50 Hz.
• Hot wire ammeter: Works on the heating effect of the current. A wire heats up and expands, and the expansion moves the pointer. The heating effect depends on I²R, so it does not depend on the direction or the frequency of the current. It therefore works on both a.c. and d.c., even at high frequencies, and reads the r.m.s. value.

3. Solution

• A and B are eliminated because they respond only to d.c.
• C is eliminated because it becomes inaccurate at high frequency.
• D is correct because the heating effect is independent of frequency.

4. Answer The hot wire ammeter, option D.

5. Tip Remember: "Hot wire = heating effect = any frequency." The only drawbacks are that it responds slowly and has a non-uniform (square-law) scale, but it is the one to pick for high-frequency a.c. in JAMB questions.