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The Riemersma critical state model is applied to a long irreversible type-II superconductor carrying ac. Voltage waveforms, energy loss per cycle, and limiting values of effective resistivity are derived. For sinusoidal current of frequency f in 10-mil wire the critical current limit of resistivity is 4.06 × 10-14f (ω.m). An equivalent circuit consisting of a variable inductance is proposed. Ex-perinlents at 4.2°K on bifilar windings of bare and copper-plated single-strand 10-mil nb-25Zr wire at 60, 400, and 800 Hz gave only fair agreement with the model with indications that the model can be improved by including surface hysteresis and eddy-current effects. Experimental values of resistivity agree more with Zar than with Wisseman, leading to the conclusion that continuous ac power applications of superconductivity are not attractive for the sake of efficiency. There remains some promise for dc energy storage for discharge into pulse power devices.