Experiments with Audiofrequencies on Superconductors

Experiments are described in which alternating and direct currents are superimposed on tin wires in the superconducting state. As the amplitude of the ac is increased, the average PD goes through a maximum and approaches an asymptotic value. The experimental curves agree with calculated curves for low frequency ac (50 cycles/sec). However, as the frequency is increased the curves change in character. The maxima increase in height and occur at large values of the ac. It is shown that this anomalous behavior can be traced to the ac skin effect, and it is concluded that experiments of this type are not suitable for a determination of the relaxation time in the phase transition from the superconducting to the normal states.