This paper reviews the present state of modern Josephson voltage standards. The presentation focuses on conventional dc standards based on underdamped superconductor-insulator-superconductor junctions and programmable standards based on overdamped superconductor-insulator-normal conductor-insulator-superconductor junctions. The current developments of ac standards on the basis of pulse-driven arrays and single flux quantum-based voltage multipliers are briefly summarized.
A description is given of the measuring system set up at the PTB for the realization of the SI volt and the determination of the conversion factor Kv by means of a voltage balance. The voltage balance comprises two cylindrical electrodes to generate the electrostatic force from the measuring voltage of 10.186 kV, and a substitution balance to measure the force by comparison with the force due to gravity acting on a 2 g mass. To achieve force equilibrium a small variable voltage is added to the constant measuring voltage. The measuring voltage is generated in a twostage 1000-fold step-up and linked up with the asmaintained volt by using a transfer standard consisting of 10 saturated standard cells connected in series. The total relative uncertainty of KV is expected to be less than 4 parts in 10' .
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