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The pressure coil circuit of a wattmeter is made of many turns of fine wire in series with a high resistance in order to

  • A.draw a small current and keep the circuit nearly resistive
  • B.produce a strong magnetic field so that the deflecting torque is as large as possible
  • C.provide damping for the moving system without a separate air chamber being needed
  • D.allow the instrument to be used on direct as well as on alternating current supplies

Answer: A

The current in the pressure coil must be proportional to the voltage and in phase with it, which needs a circuit whose resistance far outweighs its reactance. Keeping the current small also limits the error it introduces into the current coil reading.

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