A Direct Current Comparator Bridge for High Resistance Measurements

A dc comparator bridge, which is suitable for measuring resistors from a fraction of an ohm to several megohms with partper-million accuracy or better is described. An analysis of errors which may occur when measuring high-value resistors and methods used for reducing these errors to negligible proportions are included. The bridge can be connected to make either four-terminal or three-terminal measurements. The four-terminal connection is best for measuring low-value resistors. When it was used for measuring high-value resistors, a systematic bridge error was detected, which was found to be primarily due to leakage paths through the insulation, permitting current to bypass one of the measured resistors. This error is avoided by using the three-terminal connection for measuring high-value resistors. The leakage resistance is then in parallel with a low resistance winding rather than one of the measured resistors. The bridge has automatic reversal and a chart recorder output, which continuously indicates the deviation from the set point. The comparator is adjustable in steps of 10-' of full scale, and measurements can be made at ratios up to 11.111/1.