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