A.C. Ammeters

Author: J.B. Hoag

An alternating current can be measured by means of a hot-wire ammeter provided the frequency is not so high that the small capacities in the meter bypass the current around the hot wire.

A better r.f. current-measuring meter of the direct reading type is the so-called thermocouple-galvanometer or thermocouple-ammeter (or milli-ammeter).

Fig. 5 C. Principle of the thermocouple-ammeter used to measure radiofrequency currents.

In this instrument (Fig. 5 C), a very fine resistance wire is heated by the r.f. current flowing through it. A pair of wires of dissimilar metals, called a thermocouple, is attached to it and generates a small d.c. voltage which, in turn, operates a d.c. meter of ordinary design.