use of the full machinery of quantum mechanics. At the time Bardeen was among the few who recognized the importance of this theory. The Londons had fled Hitler’s Germany. Settling temporarily in Oxford, they collaborated in formulating a theory of superconductivity designed to explain the surprising result, reported in 1933 by Walther Meissner and Robert Ochsenfeld, that superconductors expel magnetic fields. This phenomenon implied that the transition between the normal and superconducting state of a metal is reversible and can thus be described by thermodynamics.

The Londons devised two equations relating the superconductor to the electric and magnetic fields. The vanishing of the resistance observed by Meissner and Ochsenfeld followed from the second equation. What the Londons realized, and expressed in their second equation, is that what is proportional to the electric field in a superconductor is not the current, as in the normal case, but the change in the current with time. If there is no electric field, the change in current will be zero, and an existing current will flow forever.

Solving the “London equations” together with Maxwell’s equations for the electric and magnetic fields yielded the observed experimental results, for instance, that the magnetic field decays exponentially as it enters the superconductor, with a penetration depth between 10^–6 and 10^–5 cm. Moreover, the current flow in the penetration layer near the surface shields the interior from the external field, the so-called “Meissner effect.” The Londons’ assumption that the normal current (which exists along with the supercurrent and satisfies the usual Ohm’s law of resistance) is short-circuited by the superconducting current under steady-state conditions explained how superconductors respond to electromagnetic waves.

Attempting to place this theory into a quantum-mechanical framework, the Londons suggested that there is “rigidity” of these ground-state wave functions in the presence of a magnetic field. In other words, if one applies weak fields, there is no effect on the wave function. But if one applies a sufficiently large magnetic field, the system ceases to be superconducting. An analogy would be pounding on a table. If one pounds gently, nothing happens because the table is rigid. But if one applies great force, the table will break.

Why should the wave function be rigid? The Londons said it was so because there exists an energy gap in the electronic structure