A prototype low temperature dc ratio transformer is described. This transformer, which uses a new technique for the precise equalization of current ratios, provides dc ratios which have accuracies of the order of 0.01 ppm and may be calibrated to 0.002 ppm. The ratio transformer may be applied in low temperature current and resistance ratio measurements of high accuracy and may, without modification, be used as a sensitive current detector.

The techniques used for the monitoring of the NSL standard of emf using the ac Josephson effect are reported. The standard has been monitored since 29th May 1970 and the reported measurements span a period of 482 days. These measurements have predominantly used niobium point contact Josephson junctions operated at the 1 mV level.The mean of the set of standard cells comprising the NSL standard has been determined in terms of a 'Josephson frequency'. The value of this frequency a t the time of the NBS-NSL volt transfer was 492.590 64 THz and a value of 2 e/h 7 483.593 80 = 0.1 ppm THz/V\-sI,.has been obtained for this period.

A cryogenic ac Josephson effect emf standard has been developed in which all precise measurements are carried out in a superfluid helium bath. This standard uses a superconducting current comparator having a ratio accuracy of better than 1 in 109 for the calibration of a resistive potential divider. The cryogenic emf standard has a precision of 1.1 in 108 and corroborates the results obtained from an ac Josephson effect standard using conventional room temperature techniques.