The attraction type of MI instrument depends on the attraction of an iron vane into a coil carrying current to be measured. fig 1 shows a MI instruments of attraction type.A soft iron vane IV is attached to the moving system.when the current to be measured is passed through the coil C,a magnetic field is produced.This field attracts the eccentrically mounted vane towards it.Thus,the pointer PR,which is attached to the spindle S of the moving system is deflected.The pointer moves over a calibrated scale.
The control torque is provided by two hair springs S1 and S2 in the same way as for a PMMC instrument;but in such instruments springs are not used to carry any current.The damping torque is provided by the movement of a thin vane V in a closed sector-shaped box B.
If the current in the fixed coil is reversed,the field produced by it reverses,so the polarity induced on the vane reverses.Thus,whatever be the direction of current in the coil,the vane is always magnetized in such a way that it attracted into coil.The force of attraction is proportional to the square of the current,provided that the moving iron vane is not magnetically saturated.Hence,such instruments can be used both for direct and alternating currents.Attraction-type MI instruments are not commonly used in practice.
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