  A model, based on classical mechanics and thermodynamics and valid for
\textit{all} superconductors, is devised to investigate the properties of the
\textit{current-driven}, superconducting to normal transition. This process is
shown to be reversible. An original derivation of the BCS variational procedure
is given. Two different critical temperatures are introduced. The temperature
dependence of the critical current is worked out and found to agree with
observation. The peculiar transport properties of high-$T_c$ compounds in the
normal state and old magnetoelastic data are also interpreted within this
framework. A novel experiment is proposed to check the relevance of this
analysis to high-$T_c$ superconductivity.
