A very low thermal EMFs computer-controlled scanner

Some National Metrology Institutes have shown an interest to build a copy of our very low thermal electromotive forces (EMFs) scanner since we designed it in our laboratory a few years ago. This device has been developed to calibrate up to 12 Zener-based voltage standards by comparison to a programmable Josephson voltage standard but can be implemented in any set-up that requires electrical contacts with a repeatability of the residual EMFs at the nanovolt level. This paper describes the technical details which explain why this device still shows an impressive robustness in its metrological characteristics after one hundred of thousands connections since its first installation. We present the result of a statistical analysis of a long series of measurements using the Allan variance, which demonstrates sub-nanovolt residual thermal EMFs.