No 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.

The relative ac-dc differences of a group of multijunction thermal converters (MJTC's) have been determined over the frequency range 30 Hz-10 kHz. These MJTC's are of different ranges and were obtained from several sources. Differences were observed at low frequencies when converters of various ranges were intercompared. For voltage measurements, the use of matched resistors in series with the M JTC heater resistors greatly reduced these errors and contributed to the reduction of other errors as well. It is believed that the average ac-dc difference of this group is less than 0.3 ppm at 160 Hz and 0.5 ppm up to 10 kHz.

No abstract

This investigation required a complete redetermination of the ac-dc differences of the NBS ac-dc transfer standards, from 20 Hz to 100 kHz. It involved the following steps: 1. Setting up a new and considerably more accurate set of primary ac-dc standards, based on multi junction thermal converters (MJTC's) and a precise emf comparator for comparing pairs of standards, 2. Using them to evaluate 5~mA single junction thermoelements (TE's) and a 10-V thermal voltage converter (TVC), up to 10 kHz, 3. Extending the frequency range by complex "bootstrap" techniques that were developed for this, and justifying these techniques. Extending the current and voltage ranges of sets of reference-standard TE's and TVC's by appropriate build-up methods, up to 20 A and from 1 to 500 V, 5. Using these reference standards to evaluate the NBS working standards that serve for calibrating clients' standards, 6.