  An electric current generates a magnetic field, and magnetic fields cannot
exist in the interior of type I superconductors. As a consequence of these two
facts, electric currents can only flow near the surface of a type I
superconducting wire so that the self-field vanishes in the interior. Here we
examine how an electric current flowing through the entire cross-section of a
normal conducting wire becomes a surface current when it enters a
superconducting portion of the wire. This geometry provides insight into the
dynamics of magnetic flux expulsion that is not apparent in the Meissner effect
involving expulsion of an externally applied magnetic field. It provides clear
evidence that the motion of magnetic field lines in superconductors is
intimately tied to the motion of charge carriers, as occurs in classical
plasmas (Alfven's theorem) and as proposed in the theory of hole
superconductivity \cite{holesc}, in contradiction with the conventional
London-BCS theory of superconductivity.
