Experiments at IEN on the ac Josephson effect monitoring of the unit of voltage

A system using niobium point-contact Josephson junctions held by a finely-adjustable spring mechanism and irradiated at 10.7 GHz has been investigated as a means of monitoring the Italian national voltage unit. Measurements were carried out at the 46th step of the junction characteristic. The ratio between the Josephson reference voltage and the e.m.f. of a standard cell was determined by balancing the two voltages against the drops across the potential terminals of two standard resistors with a resistive ratio of 1:1000, placed in still oil bath and fed by a stabilized current of about 1 mA. Preliminary results indicate a weighted mean value of 2e/h=(483,594.24 plus or minus 0.24) GHz/V IEN(73). Various modifications to the system under consideration as means of reducing the unacceptably high uncertainty in the measurement of 2e/h are discussed.