Standard-Cell Intercomparison Using a Calculator-Controlled Galvanometer in an Integral-Control Feedback System

The ability of integral-control feedback to produce a zero-error steady state, in response to a step input, is exploited for the purpose of automatically nulling a galvanometer in a standard-cell intercomparison system. The precision of the null is determined by the quantization error of an analog-to-digital converter and is about ±0.1 pA. As the zero-error property of integral control is independent of gain, reducing the gain after the null is obtained can reduce system noise without affecting the null. Changes in gain are conveniently programmed into the calculator incorporated into the system. Mechanical vibration is the dominant component of the 2-nV standard error of the intercomparison.