A simple thermoregulated enclosure for standard cells

Four new thermoregulated enclosures, each containing four high-quality standard cells, have been built and placed in service at the BIPM. The mechanical and electrical design, and the inclusion of many practical features, make these enclosures particularly well adapted for standard cell maintenance. The largest observed day-to-day fluctuation in temperature has been 0.06 mK: the long-term stability, at present apparently limited by the sensitivity of the electronic components to changes in ambient humidity, can be better than +or-1 mK per year. After a period of stabilisation, the standard deviation of the value of the EMF of a cell about its linear drift, is no more than 10 nV. A novel feature of the mechanical design makes these enclosures particularly attractive for transfer standards of voltage.