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 is 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-Ω 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 10⁹ at 1 Hz. The AC behavior of 1-Ω DC standard resistors from different manufacturers is studied using this bridge. The time constant of these resistors is on the order of 1 μs. 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 10⁹. At 1 Hz, the AC-DC difference due to the Peltier effect is found to be less than a few parts in 10⁹, with the exception of two particular resistors for which the difference was on the order of one part in 10⁸