A Direct-Reading AC Comparator Bridge for Resistance Measurement at Power Frequencies

A current comparator technique for the measurement of resistance at 50-60 Hz with an accuracy of better than 10 ppm is described. The current in the unknown resistor is compared, using the current comparator, to the current in a reference resistor for the inphase component and to the current in a reference capacitor for the quadrature component or phase defect. Six-digit resolution is provided for both components. The bridge is direct reading in either per-unit resistance or conductance. Nominal ratios of 1,2,5,10,20,50, and 100 are available. An auxiliary two-stage current transformer provides extension of the ratio range to 10 000 and also of the maximum applied current to 100 A. The bridge is suitable for measuring resistances from 10 m¿ to 1 M¿.