Recent advances in Josephson voltage standards

Recent advances of a new generation of Josephson EMF standards are described which are characterized by cryogenic operation of a resistive divider and a sensitive null detector. The advantage of using nonresonant in-line tunnel junctions and the results of resistance alloys at low temperatures are given. An AIMg5 alloy shows a temperature coefficient of resistivity of 1 part in 10⁷/K between 1.5 and 5 K. The magnetoresistance ratio was measured to 10⁻² for a magnetic induction of 15 T. Three different methods for the calibration of cryogenic resistive dividers are discussed comprising the series-parallel ratio technique, the superconducting current comparator bridge, and the low frequency inductive voltage comparator. The long-term stability of a resistance ratio of 320:1 by using the AlMg5 alloy has been measured at 4.2 K which showed a small drift of less than 5 × 10⁻⁹/h. Results of the recent intercomparison of Josephson EMF standards on the 1-V level are given. Close agreement was obtained to 6 parts in 10⁸ between the BIPM and the PTB-1 Josephson measuring systems which use conventional room-temperature techniques. The difference between the routine Josephson volt standard PTB-1 and a new developed cryogenic Josephson EMF standard PTB-2 is only 9 parts in 10⁹. All results are within the range of the combined (σ) uncertainties of the two standards, respectively. The use of a cryogenic thin film divider in conjunction with a small in-line tunnel junction and SQUID null-detector lead to a significantly reduced size of the cryogenic component parts.