Twenty years of SI ohm determinations at NML

The National Measurement Laboratory (NML) of Australia has maintained an absolute standard of resistance based on a calculable capacitor for more than 20 years. Since its inception, various developments have been incorporated which contribute to a reduction in the overall uncertainty. A reassessment of uncertainties gives a figure now of 6.2 in 10⁸ for the ohm. A systematic error, introduced in 1974, has been identified, and a corrrection of −0.09 ppm should be made to resistances expressed in terms of the NML ohm since that time.