Recent NPL Work on the AC Josephson Effect as a Voltage Standard

The experimental equipment and procedure used in the latest National Physical Laboratory (NPL) ac Josephson effect determinations of 2e/h are described. The most recent value, obtained in April 1972, is 483 594.00 (±0.10) GHZ/VNPL and this, together with earlier results, provides information concerning the stability of the NPL maintained voltage standard over the past two years. To obtain a further increase in precision, new equipment is being developed for the measurements in which the resistive divider and null detector are maintained at cryogenic temperatures.