The principle of the moving iron movement is based on the ferromagnetic force effect. Inside a coil there is a fixed and a movable iron plate.
In the moving-iron movement, when the measuring current is applied, the iron plates inside the coil are magnetized in the same direction and repel each other. The direction of the force, and thus the direction of the torque, is independent of the direction of the current. For this reason, moving iron instruments are suitable for measuringdirect current and alternating current up to approx. 300 Hz. With any signal form, the effective value is displayed because the torque depends on the square of the measuring current. Unlike moving-coil instruments, moving-iron movements are hardly ever used in multimeters or other analog measuring instruments and have largely been replaced by digital measuring instruments.
Moving iron instruments are characterized by a robust design, they are equally suitable for measuring DC and AC voltage, have a direct connection to the current and voltage transformer and are relatively insensitive to short-term overload.