A computer-controlled current-comparator-based four-terminal resistance bridge for power frequencies

A current comparator technique is applied to four-terminal resistance measurements for obtaining a highly accurate AC resistance bridge at power frequencies of 50 to 60 Hz. Active circuits are used to establish equal voltage drops between the potential terminals of the two resistances being compared. The bridge is suitable for measuring resistances from 10 μΩ to 100 kΩ. A cascading technique using two two-stage current transformers provides extension of the ratio range to 100000000 with a maximum applied current of 10000 A. The bridge features measurement with a resolution of one part in 10⁷. The total combined uncertainty (2 σ) of the bridge, including the range extenders at power frequencies, is estimated to be less than 5 μΩ/Ω