A power-frequency current-comparator-based direct-reading four-terminal resistance bridge

A current-comparator technique is applied to four terminal resistance measurements for obtaining a highly accurate direct-reading ac resistance bridge at power frequencies. Active circuits are used to establish equal voltage drops between the potential terminals of the two resistances being compared The bridge features direct reading with six-digit resolution in either per-unit-resistance or conductance. Nominal ratios of 1, 2, 5, 10, 20, 50, and 100 are available. A cascading technique using two two-stage current transformers provides extension of the ratio range to 10 000 000 with a maximum applied current to 10 000 A. The ratio uncertainties of the current comparator at power frequencies, including the range extenders, are less than 5 x 10(6) in both magnitude and phase. The bridge is suitable for measuring resistances from 10 mu Omega to 10 k Omega.