Moving iron instruments indicate the RMS value of any AC waveform applied. Consequently, such meters would normally have a non linear scale, but the iron parts are usually modified in shape to make the scale fairly linear over most of its range. The deflection of a moving iron meter is proportional to the square of the current. As alternating or direct current flows through the coil and induces a magnetic field in both vanes, the vanes repel each other and the moving vane deflects against the restoring force provided by fine helical springs. The iron element consists of a moving vane attached to a pointer, and a fixed vane, surrounded by a coil. This type of meter responds to both direct and alternating currents (as opposed to the moving-coil ammeter, which works on direct current only). The moving-iron meter was invented by Austrian engineer Friedrich Drexler in 1884. Moving iron ammeters use a piece of iron which moves when acted upon by the electromagnetic force of a fixed coil of wire.
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