Meißner effect
The active expulsion of magnetic flux from the interior of a superconductor on cooling below T_c in an applied field. The signature that superconductivity is a distinct thermodynamic phase, not mere zero resistance.
Definition
The Meißner effect is the experimental fact that reshaped the understanding of superconductivity. Before 1933, superconductors were understood as "perfect conductors": materials whose electrical resistance drops to zero below a critical temperature T_c. By Lenz's law, a perfect conductor cooled in an applied magnetic field should trap whatever flux was passing through it at the moment of the transition — the currents needed to oppose flux changes could flow indefinitely, freezing the flux in place. A perfect conductor first cooled to zero resistance and then placed in a field should exclude the new field, but a perfect conductor cooled in an already-applied field should retain it.
Walther Meißner and Robert Ochsenfeld in October 1933 discovered that superconductors do not behave this way. Cooling tin and lead cylinders through T_c in an applied field, they watched the flux actively expelled from the interior as the transition was crossed. The state of the superconductor, they showed, depended only on temperature and external field — not on history. Flux expulsion happens whether you apply the field first and cool, or cool first and apply the field: the equilibrium state below T_c has zero B in the interior, full stop. Superconductivity is therefore not perfect conduction; it is a thermodynamic phase of matter whose defining property is perfect diamagnetism, χ = −1.
The depth to which the field penetrates before being expelled is the London penetration depth λ, typically 10⁻⁷ metres. Superconducting surface currents flow in a thin shell of that depth, producing a magnetization exactly opposing the applied field and driving B → 0 in the bulk. Above a critical field H_c the superconducting state collapses entirely and the material reverts to normal conduction; between zero and H_c, the Meißner state persists. The London brothers' 1935 phenomenological theory and BCS theory in 1957 both have flux expulsion built in from the start — any microscopic model of superconductivity must predict the Meißner effect, and any theory that does not fails the experimental test immediately.