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The braking magnet of an energy meter is

  • A.an electromagnet excited from the pressure coil circuit of the meter
  • B.a friction pad bearing lightly against the edge of the aluminium disc
  • C.an air vane attached to the spindle and moving in a closed chamber
  • D.a permanent magnet acting on the rim of the disc

Answer: D

Eddy currents induced in the moving disc by the permanent magnet produce a torque opposing the motion and proportional to the speed. The disc therefore settles at a speed at which braking torque equals driving torque, so speed is proportional to power.

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