Maintaining the Unit of Voltage at PTB via the Josephson Effect

In Section I a report on high precision voltage comparisons between the mean EMF of the former PTB voltage standard (consisting of a group of 39 saturated Weston cells) and the Josephson reference voltage is presented. The experiments were carried out with a total uncertainty (1 σ) of 4 parts in 108. The measured rate of change of the mean EMF during a 1¾ year period was - 1.3 X 10-7 V per year. This voltage stability is sufficient to maintain the unit of voltage by this group of standard cells for several months until a new comparison with the Josephson reference voltage becomes necessary. Due to the effects of thermal EMF's in the millivolt circuit of the measuring system used at present, the Josephson reference voltage (≈
₃
. mV) is only stable during a short time. In Section II a prototype cryogenic voltage standard developed at PTB is described. By immersing the main measurement components into the superfluid liquid helium bath, a long term voltage stability can be achieved. These components include the cryogenic resistive divider, consisting of a new copper alloy, and the SQUID null detector. The resistance ratio of the cryogenic resistive divider of 320:1 is determined by a ten-decade inductive voltage comparator operating at 84 Hz. The effects of power dissipation introduce only errors of second order because the currents in the calibration mode and the measurement mode are the same.