ACCURATE AC MEASUREMENTS OF STANDARD RESISTORS BETWEEN 1 AND 20 HZ

A new version of an ac bridge for low-frequency (1-20 Hz) measurements of resistance ratios has been designed. The bridge is based on an alternating current comparator operating at room temperature. The current comparator uses a single magnetic core of high permeability provided with a tuned detection winding enclosed in electrostatic and magnetic shields. Comparators with ratios equal to 1/1, 100/1, 64.53/1, and 129.06/1 were built, allowing the calibration of 1 and 100 OMEGA resistance standards in terms of the quantized Hall resistance. Ratio windings are formed by the series connection of strands from a set of individually guarded cables, a technique which provides low ratio error, not exceeding a few parts in 10(9) at 1 Hz. The ac behavior of 1 OMEGA dc standard resistors from different manufacturers was studied using this bridge. The time constant of these resistors is on the order of 1 mus. If the frequency does not exceed 1 Hz, the ac-dc resistance difference due to eddy currents in the metallic can enclosing the resistors is lower than one part in 10(9). At 1 Hz, the ac-dc difference due to the Peltier effect was found to be less than a few parts in 10(9), with the exception of two particular resistors for which the difference was on the order of one part in 10(8).