The fundamental physical constant $\frac{e}{h}$ has been experimentally determined with high accuracy, using the ac Josephson effect in systems of weakly coupled superconductors (Josephson junctions). The measurement depends on the validity of the Josephson frequency-voltage relation $\nu =\frac{2eV}{h}$, which relates the frequency of the oscillating supercurrent between two weakly coupled superconductors to the potential difference between them. The frequency-voltage ratio was measured using two effects: (a) constant-voltage steps induced in the dc current-voltage characteristics of Josephson junctions by microwave radiation and (b) microwave radiation emission by Josephson junctions biased at nonzero voltages. The ratio was found to be independent of the type of superconductor and the type of Josephson junction used, nonlinear harmonic and subharmonic effects, temperature, magnetic field, and microwave power and frequency. These results justify confidence in the general validity of the Josephson frequency-voltage relation and in the identification of the frequency-voltage ratio with $\frac{2e}{h}$. The final experimental result is $\frac{2e}{h}=483.5976\mathrm{}\pm 0.0012$ MHz/μ${\mathrm{V}}_{\mathrm{nbs}}$ (standard deviation) referred to the volt as maintained by the U. S. National Bureau of Standards prior to January 1, 1969. The result in terms of the NBS volt as maintained after this date, ${\mathrm{V}}_{69\mathrm{nbs}}$, is 483.5935 ± 0.0012 MHZ / ${\mathrm{\mu }}_{69\mathrm{nbs}}$.

• Received 15 July 1968

DOI:https://doi.org/10.1103/PhysRev.177.639