ELECTRICAL-RESISTANCE STANDARDS AND THE QUANTUM HALL-EFFECT

This review of electrical resistance standards begins with a descripti on of classical standard resistors and their limitations. Methods of c omparing resistance are described; these include bridges based on cryo genic current comparators capable of achieving statistical uncertainti es approaching one part in 10(10) in the measurement of resistance rat ios. Such reproducibility is nearly two orders of magnitude smaller th an the overall uncertainty of the most accurate determinations of the ohm from its SI definition via the calculable capacitor. The quantum H all effect can provide an invariable reference standard of resistance linked to the fundamental physical constants. Many factors, however, l imit the accuracy of practical realizations of quantized Hall resistan ce standards. Ultimately, the accuracy of a specific realization must be confirmed by comparison with similar standards; methods for doing t his and the resulting agreement are presented. The ac techniques used in the determinations of the SI ohm by means of the calculable capacit or are now being applied to accurately link the quantized Hall resista nce to the impedance of standard capacitors and thereby to provide a n ew reference standard of capacitance. (C) 1998 American Institute of P hysics.