In repulsion type instrument the force of repulsion is approximately proportional to

In repulsion type instrument the force of repulsion is approximately proportional to

Right Answer is:

Square of current


In a repulsion type instrument, the force of repulsion is approximately proportional to the square of the current.

Moving iron instruments are used to measure current and voltage. They are made as ammeters and voltmeters and can measure both ac and dc currents and voltages respectively. Two types of moving iron instruments are manufactured. One type is called attraction type and the other type is called repulsion type.

Repulsion type Ammeter

  • In repulsion type, a fixed vane or rod is also used.
  • Controlling 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.

The fixed coil carries the current or current proportional to the voltage to be measured. On the inner surface of the coil, a specially shaped fixed iron piece is attached.  The spindle is attached to another piece of iron in the form of a fin. This is the moving iron. The current passing through the coil produces a magnetic field. In this magnetic field, both the fixed iron and the moving iron are placed. Both these iron pieces will be similarly magnetized due to the influence of the magnetic field. This will create a force of repulsion among the two pieces of iron. One piece of iron is fixed, the other piece of iron will move which will create deflecting torque. This deflecting torque has to be opposed by a control torque such that under steady deflection,

Td = Tc.

If Td is more than TC, the pointer will move. Its movement will stop when the deflecting torque, Td, equals the control torque, TC. Since the magnitude of deflecting torque depends upon the magnetism of the two pieces of iron and since this magnetism is produced by the current to be measured. we can write

Td ∝ I2

And Tc ∝ θ  as more the deflection θ,  more is the controlling torque.

Moving iron instruments have their deflection is proportional to the square of the current flowing through the coil. These instruments are thus said to follow a square-law response (R.M.S Value) and have non-uniform scale marking. Deflection being proportional to the square of the current, whatever be the polarity of the current in the coil, deflection of a moving iron instrument is in the same direction. Hence, moving iron instruments can be used for both DC and AC measurements.

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