Question

What value of alternating current is measured by a hot wire ammeter?

• A. High value
• B. Average value
• C. Root mean square value
• D. None of these

Hint:

Hot wire ammeters measure RMS current due to their reliance on thermal effects proportional to \( I^2 \).

Answer 1

The Correct Option is C

A hot wire ammeter measures the root mean square (RMS) value of an alternating current (AC) because it operates based on the heating effect produced by the current passing through a wire. The heating effect (Joule heating) is proportional to the square of the current \( I \), i.e., \[ P \propto I^2. \] For an alternating current, the instantaneous current varies with time, but the wire heats up according to the average power dissipated, which depends on the mean of the square of the current. Therefore, the hot wire ammeter effectively responds to the RMS value of the AC, which represents the equivalent direct current (DC) value that would produce the same heating effect in the wire.