Thermocouple-type ammeters for use at very high frequencies

THE measurement of currents at the higher frequencies is complicated by the ever-magnified inductances and capacitances associated with the measuring equipment. On direct currents, permanent-magnet moving coil instruments are used extensively, and here the main consideration is the series resistance of the instrument. Instruments for lower frequency alternating currents generally make use of the electrodynamic or some form of moving iron measuring system. Here again the series resistance of the instrument is important but, in addition, the series inductance of the necessary coils becomes a factor. As the frequency is increased the effects of the inductance also increase, and in the middle audio-frequencies these effects become formidable.