Thermal converters as AC-DC transfer standards for current and voltage measurements at audio frequencies

Abstract

Thermal con verters and associated equipment that are used as ac-dc transfer standards at t he National Bureau of Standards for the precise measurement of current and voltage at power and audio frequencies are described. The tandards and t h e equipment are primarily used to standardize a-c ammeters and voltmeters submitted to the Bureau for certification. Th e ac-dc transfer may be made with t hese thermal converters at currents from 1 milliampere to 50 amperes, voltages of 0.2 to 750 volts, with an accuracy of 0.01 percent at frequencies from 25 to 20,000 cycles per second. The special tests to ins ure t he required accuracy of the transfer standards are described, and the results are presented. A number of factors that limit the transfer accuracy of thermal con verters have been discovered, and the results of special tests and theoretical work to evaluate these factors are discussed. The solu t ions, by an approximation method, of certain pertinent nonlinear differential eq uations governing the heating of a conductor by an electric cun'ent arc given.