On the measurement of alternating electric currents of high frequency

Abstract
Both from the scientific and practical point of view the accurate measurements of alternating current, voltage and power at high frequencies is of considerable importance. For example, in the domain of pure science the study of energy losses in dielectrics at high frequencies is most interesting, while on the practical side, in radiotelegraphy, there is a wide field for improved methods of measurement. At frequencies from 100,000 to 2,000,000 ∼ per second by the methods at present in use it is comparatively easy to measure currents of the order of 0.1 ampère, but many difficulties present themselves when we have to deal with currents of 1 to 50 or 100 ampère. We have recently investigated several ways of measuring these larger currents, and the object of the present paper is to give a description of the more satisfactory methods and the results obtained by them. Thermal Instruments.-the most common method of measuring a high frequency current is to pass it through a conductor and observe the rise of temperature of this as shown by its expansion or change of resistance, or by means of one or more thermojunctions in contact with it or near it. The last method, that of a better and separate thermopile, was investigated by one of us many years ago and was later to measure the voltages induced in small search coils by alternating magnectc fields. The separate heater system was used by Mr. Duddell in his beautiful application of Mr. Boy's radiomicrometer to current measurement;and his thermo-galvanometer still represents the high-water mark in efficiency. While, however, the self-contained thermojuction and moving coil no doubt give the highest efficient, the use of a thermopile connected to a separate galvanometer has several advantages, especially in laboratories where economy of instruments has to be considered. A variety of heaters and thermopiles can be used with a single galvanometer, and both the sensitivity and the promptness can usually be altered within pretty wide limits by adding resistance to the galvanometer circuit.

This publication has 0 references indexed in Scilit: