A-C Indicating Meters

The moving iron-vane type is the most common a-c meter. In it induced eddy currents are used to produce magnetic force on a structure bearing a pivoted pointer and a thin iron element called a vane. The vane has no coil. The stationary magnetic field is produced by a single current-carrying coil surrounding both the fixed metal element and the pointer movement. This coil is so arranged that its own field induces a field in the moving vane and in addition generates attractive or repulsive magnetic forces with respect to its own self-produced magnetic field. Deflection is basically proportional to the current through the main coil.

Moving iron-vane meters usually have relatively low impedance and are simple and inexpensive. They measure either voltage or current, but their use must be restricted to the frequency for which they are designed.

Rectifier type meters utilize PM d-c movements actuated by current developed from rectifying the applied a-c being measured. Rectifier elements mounted within the meter case may be copper oxide, selenium, germanium, or silicon. The developed d-c is proportional to the applied a-c while the rectifiers and associated circuitry are designed for operation over as wide a band of frequency as possible.