  Old susceptibility data, measured in superconducting materials at
low-frequency, are shown to be accounted for consistently within the framework
of a recently published\cite{sz1} analysis of the skin effect. Their main merit
is to emphasize the significance of the skin-depth measurements, performed
\textit{just beneath} the critical temperature $T_c$, 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.
