Problems associated with a.c.-d.c. transfer devices for the precise measurement of alternating currents and voltages

• Main Author: Widdis, F. C.
• Published: University of London 1959
• Subjects: 621.37
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.318196

possible transfer device for the precise measurement or standardization of an alternating current with reference to a precisely known direct current. A full method of operation has been developed and an experimental and theoretical consideration of the various sources of error carried out. The performance has been compared with that of existing a.c. - d.c. transfer devices, and it is shown that the indirectly heated thermistor is a valuable addition to this field, particularly in the measurement of very low frequencies. The actual idea of using the indirectly heated thermistor as a transfer device is not original, nor is the measurement of the thermistor characteristics themselves (section 2.5). It is believed, however, that all the work concerning the actual bridge design and operation, the analysis of its performance, and the investigation of the sources of error is original, and that, as a result of this work, this device can be confidently used for measurements of the highest precision. During the course of this investigation, comparison measurements were carried out between the indirectly heated thermistor and the vacuo-thermo-junction. Discrepancies which appeared in these measurements were ultimately attributed to limitations in the existing theory of transfer errors in vacuo-thermo-junctions, concerning the effects of Thomson and Peltier heating. This theory has been reconsidered and a new generalized theory developed which is shown to be applicable also to the indirectly heated thermistor. It is evident, that providing certain precautions are taken in the construction, the vacuo-thermo-junction is capable of the highest precision. It is believed that the new 3. theory and the supporting analysis is largely original. Some of the work on the indirectly heated thermistor has been published by the author in the Proceedings of the Institution of Electrical Engineers, Vol. 103, Part B, November, 1956.