The Moving Iron Ammeter
Moving iron ammeters use the repulsion between pieces of soft ironinside a current carrying coil to indicate and measure a current. Oneof the pieces of iron is fixed and the other is free to rotate.
The current to be measured flows through a coil around bothmagnets, producing a magnetic field proportional to the strength ofthe current.
The magnetic field magnetizes these pieces with the same polarity.Since like poles repel, the movable iron turns away from the fixedvane, moving the pointer and producing a restoring turning torquefrom the spring. The iron will contimue to turn until the restoringtorque exerted by the spring equals the torque produced by the mutualrepulsion of the soft iron pieces.
Because the polarities of the iron pieces is always the same, theywill always exert a repulsive force. If the direction of the currentchanges, so do the polarities of the magnetic pieces, AND AT THE SAMETIME. The instrument can be used to measure alternating currents,unlike moving coil meters.