Accurate Hamon-Pair Potentiometer for Josephson Frequency-to-Voltage Measurements

Abstract

Accuracy and precision of parts in 107 have been demonstrated in a potentiometer which compares dc signals in the 2 to 10 millivolts range from Josephson junctions, against the U.S. "legal volt", as maintained by a group of standard cells. The circuit comprises two (10 × 100 ohms) Hamon networks in series-to-parallel transposition, linked by a calibrated Kelvin-Varley current divider. The critical Hamoncircuit contacts are made through mercury-wetted amalgams; the entire circuit operates in smoothly stirred oil. Observations indicate the pair of Hamon boxes to be initially matched within 0.2 ppm at the operating temperature. Typical standard deviation in the day-to-day, run-to-run scatter of Josephson frequency-to-voltage ratio, measured with this instrument, is 0.3 ppm or less. Preliminary values for the observed ratio are in general accord with the quoted values of Parker et al, Petley and Morris and Finnegan et al.