The effect of different surface treatments on flux penetration and dissipation at 50 Hz in bulk niobium samples has been quantitatively investigated. It is shown that for a sufficiently smooth sample surface the onset of a.c. dissipation occurs when the peak applied magnetic field exceeds H c1 +Δ H , in striking contrast to earlier results. A comparison of experimental results with a generalized critical-state model yields good agreement.
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Easson et al. (1968) studied this question.