Thermocouple meters

Thermocouple meters convert applied current (a-c or d-c) by thermoelectric effects into deflection currents which register on a PM moving-coil couple actuated by heat generated at the junction formed by two strips of dissimilar metal. One strip carries the heat-generating input currents, the other carries heat-produced currents to the meter movement.

Thermocouple meters are used to measure a wide range of either a-c or d-c currents and give an accurate picture of the effective, or heats producting, value of a current no matter what its nature. The meters are relatively expensive and the thermal delay in converting input electricity to heat makes them sluggish.

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.

Indicating D-C Meters

Fundamentally, all meters measure current and in this sense are ammeters. When they have low internal resistance so as to produce no effect on a circuit's performance and are actuated through direct connection in series with the main current path, they are truly ammeters.

When they have high resistance so that they may be placed in shunt or directly across a voltage source they are called voltmeters. Their indication of voltage is based on the product of a known, calibrated internal resistance in series with the meter movement, and the current flowing through that resistance.