  It is generally believed that the laws of thermodynamics govern
superconductivity as an equilibrium state of matter. Here we point out that
within the conventional BCS-London description of the normal-superconductor
transition in the presence of a magnetic field, the transition cannot be
reversible, in contradiction with the thermodynamic description and with
experiments. This indicates that the conventional theory of superconductivity
is internally inconsistent. We argue that to describe a reversible transition
it is necessary to assume that charge transfer occurs across the
normal-superconductor phase boundary, as proposed in the theory of hole
superconductiviy. This provides also a solution to the angular momentum puzzle
pointed out in previous work. Our explanation can only apply if the current
carriers in the normal state are holes. An experimental test of these ideas is
proposed.
