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Damping in a dynamometer wattmeter is provided by

  • A.the viscous drag of an oil in which the whole moving system is immersed
  • B.an air friction vane
  • C.eddy currents induced in an aluminium disc attached to the moving system
  • D.a permanent magnet acting on a light aluminium former on the moving coil

Answer: B

Eddy current damping is ruled out, because a permanent magnet near the coils would distort the very field on which the reading depends. A light vane moving in a closed sector supplies the damping without introducing any magnetic disturbance.

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