An AC ammeter connected in an AC circuit reads a current i. If this AC ammeter is replaced by a DC ammeter, what will the DC ammeter read?

i/sqrt(2)
sqrt(2)*i
0.637*i
zero

Correct answer: zero

Solution

An AC (hot-wire/moving-iron) ammeter measures the rms current i. A DC (moving-coil) ammeter responds to the time-average of the current. For a pure sinusoidal AC, the average over a complete cycle is zero, so the moving-coil DC ammeter reads zero.

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