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Detection of radio-frequency currents by superconducting columbium nitride has been studied as a function of rf current, bias current, and temperature. The existence of nonlinear resistance in the zone between super and normal conduction has been observed, with specially high values of dR/dI for currents less than one ma; this is believed to be due to the effect of the magnetic field of the current superconductivity (Silsbee hypothesis). The observed values of dR/dI account in a rough quantitative way for the observed rectified potentials and their variation with rf current, bias current, and temperature at 1 Mc. Increasing rectification at higher frequencies is observed, but not yet explainable.