 Moving iron instuments are classified as:
1)Attraction type
2)Repulsion type

Basic working principle :
“ A soft iron piece if brought near the magnet gets attracted bye magnet.
”

ATTRECTION TYPE INSTRUMENT

 It consist of fixed coil C and moving iron piece D. The coil moving coil is a flat disc which is eccentrically mounted on the spindle.
 The spindle is supported between the jewel bearings .
 The spindle carries pointer which moves over a graduated scale.
 The no. of turns of the fixed coil are dependent on the range of the instrument.
 For passing large current through the coil only few turns are required.

 The controlling torque is provided by the springs but the gravity control may be used for vertically mounted panel type instrument.
 The damping torque is provided by the air friction . A light aluminum piston is attached to the moving system .
 It moves in a fixed chamber. The chamber is closed at one end. It can also be provided with the help of vane attached to the moving system.
 The operating magnetic field is very weak.
Hence eddy current damping is not used since it requires a permanent magnet which would affect or distort the operating field.

Basic working principle:
“ There is repulsion between two iron pieces as they are magnetized with the same polarity.
”

 A hollow circular coil is used and one of the iron pieces is attached inside the hollow coil.
 The other iron piece is attached to the spindle.
 The current to be measured is passed through the coil, coil produced the magnetic field.
 As both the iron pieces are placed in the same magnetic field, they are magnetized with same polarity.

 Hence the deflecting torque is produced due to the force of repulsion between two iron pieces.
 Independent of the direction of current, the deflection is produced in the same direction.
 the force of repulsion between iron pieces is directly proportional to square of current hence scale is non-uniform.
 Controlling torque is produced by either spring or gravity control.
 Damping is produced by the air friction damping.

REPULSION TYPE INSTRUMENT