A Direct-Current-Comparator Ratio Bridge for Four-Terminal Resistance Measurements

A ratio bridge has been built which compares four-terminal resistances by measuring the current ratio corresponding to voltage drop equality. A current ratio, corresponding to turns ratio, is automatically maintained by a self-balancing direct current comparator and is adjustable in part-per-million steps. The two current sources are isolated so that, at balance, there is no current in the potential circuit. The bridge can be direct reading in either resistance or conductance ratio. The bridge makes use of the ability of a self-balancing direct current comparator to measure and compare accurately the ratio of two isolated direct currents. A detailed description of the bridge is presented with an analysis of its accuracy limitations. In its optimum operating range accuracies of better than one part per million can be achieved. The bridge is particularly suited to the scaling of resistance standards from one hundred ohms to a fraction of a milliohm. It can be used at any ratio up to one thousand to one. It permits the calibration of low value shunts at full operating current with an accuracy limited only by their noise level and stability.