Moving Iron Ammeter

(a)    Soft Iron Attraction Type.

A piece of soft iron is pivoted eccentrically in front of a coil of wire. The current to be measured is passed through the coil. The soft iron piece responds to the magnetic field of the current and gets attracted into the coil irrespective of the direction of the current. The magnetization of the iron piece as well as the magnetic field is both proportional to the current. So the force of attraction is proportional to the square of the current. The iron piece comes to rest when the force of attraction on it is balanced by a counterweight (or the restoring force of a hair-spring). A pointer attracted to the iron piece then gets deflected and moves over a scale. The scale is calibrated to read the RMS value of the current directly.

(b)    Soft Iron Repulsion Type.

Two identical iron rods are placed inside a solenoid. Once of them (A) is fixed whereas the other (B) is capable of rotating about pivots. When the current is passed through the solenoid both the soft iron rods are magnetized is the same direction. Therefore, they repel each other. The magnetization of each rod is proportional to the current. So the force of repulsion is proportional to the square of the current. Also, the direction of the force of repulsion on the movable iron piece is independent of the direction of the current. The movable iron piece comes to rest when the force of repulsion of it is balanced by a counterweight or the restoring force of a hair-spring. A point attached to the movable iron piece then gets deflected and moves over a circular scale. The scale is calibrated to read the RMS value of the current directly.


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