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The deflecting torque of a dynamometer wattmeter is proportional to

  • A.the sum of the two coil currents multiplied by the rate of change of inductance
  • B.the product of the two coil currents alone, whatever their phase relation may be
  • C.the product of the two coil currents and the cosine of the angle between them
  • D.the square of the current in the fixed coil taken alone

Answer: C

Averaged over a cycle the torque is proportional to \( V I \cos\phi \), which is exactly the active power. The cosine term is what makes the instrument read power rather than volt-amperes, and why it reads correctly at any power factor.

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