  A number of facts indicating the relevance of the Darwin magnetic interaction
energy in the superconducting phase are pointed out. The magnetic interaction
term derived by Darwin is the same as the, so called, Breit term in
relativistic quantum mechanics. While this term always is a small perturbation
in few body systems it can be shown to be potentially dominating in systems of
large numbers of electrons. It is therefore a natural candidate in the
explanation of emergent phenomena---phenomena that only occur in sufficiently
large systems. The dimensionless parameter that indicates the importance of the
magnetic energy is the number of electrons times the classical electron radius
divided by the size of the system. The number of electrons involved are only
the electrons at the Fermi surface; electrons with lower energy cannot
contribute to current density and thus not to the magnetic field.
  The conventional understanding of superconductivity has always been
problematic and no really reductionistic derivation exists. The idea that the
inductive inertia, due to magnetism, is important in the explanation of
superconductivity was first advanced by Frenkel and later brought up by Welker
before it was prematurely discarded. So were theories involving Wigner
crystallization. We speculate that the answer requires the combination of a
Wigner lattice and the Darwin interaction. We point out that the Darwin
interaction can be shown to have the right order of magnitude to explain the
energy scales involved in normal superconductors. The London magnetic moment of
rotating superconductors and the Meissner effect and their connection are
discussed next. The London moment is shown to indicate that the number of
electrons involved in the superconducting condensate is such that the Darwin
interaction cannot be neglected.
