Experiments were conducted to determine the effective diffusivity for axial transport through a tube of circular cross-section of a contaminant gas in oscillatory flow. Results were compared with the theoretical predictions of Watson (1983) and found to be in excellent agreement. The experiments differ from the theoretical situation in that the oscillations are superimposed upon a steady flow due to a constant infusion of tracer gas, and a buoyancy-induced flow associated with spatial variations in gas density. The influence of both artifacts is found to be negligible.

We have measured the current-voltage (I-V} characteristics of several high-temperaturesuperconducting materials with widely difkrent morphologies {bulk Ag/Pb-Bi-Sr-Ca-Cu-0 tapes, thin films of Y-Ba-Cu-O, and melt-textured, bulk Y-Ba-Cu-0 samples). The I-V curves were taken at several magnetic fields ranging from 0 to 8 T. The measurements were carried out at three temperatures (4.2, 27, and 77 K) where the samples were immersed in liquid cryogens to ensure good thermal equilibrium. We compared our experimental results to the predictions of dissipation in superconductors made by the following physical models: modified Ambegaokar-Halperin, flux creep, vortex glass, collective flux creep, and a power law. The fits were extremely good for the first model and were not nearly as good for the others. Using the modified Ambegaokar-Halperin model, the critical current I"the normal-state resistance R",and y, which is proportional to the pinning potential U(H, T), were obtained for each material. Since the Ambegaokar-Halperin model is the only one which uniquely defines I"we conclude that its use puts this parameter on a solid physical basis.

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