The development of voltage standards based on superconductive tunnel junctions began with the 1962 prediction by Brian Josephson that the average voltage across a junction subjected to microwaves of frequency ƒ may be quantized in units of 2eƒ/h, where e and h are the elementary charge and Planck's constant. NIST vigorously pursued the realization of a Josephson voltage standard, and in 1972 the U.S. practical volt was redefined using two junctions connected in series and driven at 9 GHz to produce a quantized voltage near 10 mV. The present calibration workhorse, perfected in the 1980's, uses a series array of 20,000 under-damped junctions driven at around 80 GHz to produce quantized voltages in excess of 10 V and can be quickly programmed to produce any desired voltage. Finally, an entirely new technique, based on pulsed operation, has been adopted to create highly accurate ac waveforms.
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For more information, contact Stephanie Shaw at 301-975-2667.
Colloquia are videotaped and available in the NIST Research Library.