Standard Cell Enclosure with 20-μK Stability

Groups of standard cells are needed to maintain the National Bureau of Standards (NBS) volt between periodic reassignments via 2e/h. An enclosure to hold six standard cells has been designed and three enclosures have been assembled. The enclosures consist of four concentric aluminum cylinders separated by foamed polystyrene insulation. Two of the four cylinders are controlled, an outer one by a platinum resistance thermometer and dc amplifier and the inner one at 30°C by a second platinum resistance thermometer and an ac transformer-ratio bridge followed by a phase-sensitive detector, a dc amplifier, and a heater. The innermost of the four cylinders is a thermally lagged cell compartment. The temperature excursions over periods of at least several days were found to be less than 20 μK, thus having a negligible effect on the standard cell EMF's.