Reduction of Kelvin Ratio Arm Errors in an AC Double Bridge Using Electronic Control

The systematic error of the Kelvin ratio arm in a low-frequency alternating current bridge is a function of the known and unknown impedances, and of the impedance of the coaxial cable connecting the two standards. To decrease this systematic error, an electronic amplifier has been developed and used, so that the voltage drops at the coaxial cable are measured by a differential amplifier and fed back into the coaxial cable by means of an injection transformer. In this way, the error of the Kelvin ratio arm is reduced by the order of 500.