Movingiron

Moving-iron is a class of electrical measuring instruments that use the interaction between a soft iron moving element and a magnetic field to produce deflection of a pointer. The moving element, usually a vane or small iron piece, is allowed to rotate within a magnetic field produced by a coil and a fixed magnetic circuit, which may be a permanent magnet or an auxiliary electromagnet. In attraction-type instruments the iron piece is drawn toward a fixed pole, while in repulsion-type instruments two iron pieces repel each other as they are magnetized by the coil.

Operation and characteristics: When current flows through the coil, the iron piece becomes magnetized and experiences torque in the surrounding magnetic field. The resulting force moves the pointer against a restoring spring. The deflection is generally proportional to the square of the current, so the instrument indicates current magnitude rather than direction. This makes moving-iron meters suitable for alternating current (AC) as well as direct current (DC) measurements, though the scale is not perfectly linear.

Construction and use: Moving-iron meters are robust, simple, and inexpensive compared with moving-coil meters. They are commonly used as voltmeters and ammeters in older equipment, panel meters, and educational instruments. They tend to be relatively insensitive to vibration and orientation, but their accuracy and temperature stability are inferior to moving-coil instruments, and they require calibration for best accuracy. They cannot indicate current polarity, reflecting their magnitude-only response.

In summary, moving-iron instruments offer rugged, economical operation for measuring current and voltage, with a fundamental trade-off in precision and linearity compared to moving-coil devices.