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A moving coil instrument is unsuitable for direct use on alternating current because

  • A.its pointer would read the average, which is zero
  • B.its coil would burn out at any alternating current
  • C.its permanent magnet would be demagnetised immediately
  • D.its scale would become cramped at the lower end

Answer: A

Averaging a sinusoid over a cycle gives nothing, so the pointer sits at zero while quivering slightly. Adding a rectifier makes the movement usable on alternating current at the cost of a waveform-dependent calibration.

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