Old susceptibility data, measured in superconducting materials at low-frequency, are shown to be accounted for consistently within the framework of a recently published [1] analysis of the skin effect. Their main merit is to emphasize the significance of the skin-depth measurements, performed just beneath the critical temperature Tc, in order to disprove an assumption, which thwarted any understanding of the skin-depth data, achieved so far by conventional high-frequency methods, so that those data might, from now on, give access to the temperature dependence of the concentration of superconducting electrons.
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