ac Josephson current as a function of voltage

The superconductive current flowing through niobium point contacts was measured as a function of applied voltage using a bridge circuit. With the assumption that the dc component of the superconductive current is proportional to the amplitude of the ac Josephson current, the latter was determined as a function of the voltage applied to the contact. For eV greater than the gap energy, the results agree qualitatively with theory. A maximum in the superconductive current (the Riedel peak) was observed in the superconducting current-voltage curves and in the curve of IF signal amplitude versus voltage in experiments involving the mixing of two UHF signals.