Development of a Standard Cell Reference Group with Microprocessor-Controlled Intercomparisons

An automatic system is described to intercompare the standard cells of a reference group. A microcomputer controls the selection of cells or external sources, the electromotive force (EMF) difference measurement, and the protection devices. The programmed cell selection is performed by actuating suitable low-thermal rotary switches driven at limited speed. The EMF difference is measured through an automatic potentiometric device at nanovolt level. Protection is provided by both software and hardware. The system designed is intimately connected to a new thermostatic air bath and aimed at improving the uncertainty of the comparisons to the limit imposed by thermal noise to a few nanovolts. It is intended both for the maintenance of the voltage unit alongside a Josephson standard and for first-level dissemination of the same unit. Preliminary experimental results on the performance of the automatic system are reported.