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True or false: an integrating digital voltmeter set for fifty hertz rejects sixty hertz interference equally well.

  • A.True, integration rejecting interference at any frequency whatever
  • B.True, provided the reading is taken over several complete cycles
  • C.True, since twenty milliseconds contains more than one cycle of sixty hertz
  • D.False, twenty milliseconds is 1.2 periods of sixty hertz

Answer: D

Twenty milliseconds is 1.2 periods of sixty hertz, so the average over it is not zero and the rejection is poor. Instruments intended for both regions offer a line frequency selection that changes the integrating period accordingly.

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