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Attraction type and repulsion type moving coil instruments?
Moving-iron instruments are generally used to measure alternating voltages and currents. In moving-iron instruments the movable system consists of one or more pieces of specially-shaped soft iron, which are so pivoted as to be acted upon by the magnetic field produced by the current in coil.

There are two general types of moving-iron instruments namely:

1. Repulsion (or double iron) type
2. Attraction (or single-iron) type


The brief description of different components of a moving-iron instrument is given below:

  • Moving element: a small piece of soft iron in the form of a vane or rod.
  • Coil: to produce the magnetic field due to current flowing through it and also to magnetize the iron pieces.
  • In repulsion type, a fixed vane or rod is also used and magnetized with the same polarity.
  • Control torque is provided by spring or weight (gravity).
  • Damping torque is normally pneumatic, the damping device consisting of an air chamber and a moving vane attached to the instrument spindle.
  • Deflecting torque produces a movement on an aluminum pointer over a graduated scale.


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