Frequency Errors in Radio-Frequency Ammeters

This paper discusses from a theoretical standpoint the various types of frequency errors in thermal ammeters, describes methods of calibration of current measuring instruments at radio frequencies, and furnishes some data as to the accuracy of certain commonly used instruments at frequencies up to 100 megacycles. Frequency errors are determined in the low range instruments and in the high range instruments (ten amperes full scale or larger) at the lower radio frequencies (below thirty megacycles) by means of specially constructed tungsten filament lamps, a photoelectric cell, and a direct-current instrument for measuring the output of the cell, this equipment being used as a transfer device in comparing two currents, one at sixty cycles and the other at the applied radio frequency. Frequency errors in the high range instruments above thirty megacycles are determined by a substitution method. By this means, the reading of an ammeter under test may be compared with that of another instrument in which the frequency error has already been determined (with calibrating lamps, etc.), using an additional instrument in the circuit to insure that no change in circuit current has occurred.