Abstract:
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The National Research Council of Canada, in association with NMIA in Australia and BIPM in France, is establishing a calculable capacitor that represents a third generation of design. The calculable capacitor was original developed in the 1960s by Thompson in Australia and refined by others, notably Cutkosky at NBS. Its development caused one of the first major changes in the realization of SI electrical units in that it accurately derived a capacitance, only in terms of length and the permittivity of free space. Between 1969 and 1990 the calculable capacitor was the practical reference of the world's capacitance and resistance standards. While the discovery of the quantum Hall effect and its subsequent use as a resistance standard since1990 has lessened our dependence on calculable capacitors, they still remain important and independent determinations of the capacitance unit, the farad. There are only a few operating calculable capacitors in the world and only a couple that can achieve uncertainties of order 5x10-8. This has been primarily due to inherent limitations in previous designs coupled with the difficulties in assessing a number of critical influences. While the dimensional tolerances of the new calculable capacitor are extremely demanding it is expected to achieve uncertainties of ? 10-8 and also has other properties that are unique. We will describe the design considerations of this new design, outline some target specifications and uncertainties and review the progress to date. We will also present the contributions that such a calculable capacitor can make to SI units and to the determination of fundamental constants in light of the recently proposed changes to the SI by Mills et. al. [1]. 1. "Redefinition of the kilogram, ampere, kelvin and mole: a proposed approach to Implementing CIPM recommendation 1(CI-2005)", I. Mills, P. Mohr, T. Quinn, B. Taylor and E. Williams, Metrologia, 2006
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