= Meissner effect
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= Meissner-Ochsenfeld effect
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The <Meissner effect> is the equilibrium expulsion of a sufficiently weak, static <magnetic field> from the bulk of a material exhibiting <superconductivity>. Expulsion includes a <magnetic field> that was already present before cooling: zero <electrical resistance> alone does not predict this. The <Field-cooled Meissner experiment> tests that distinction. A finite <London penetration depth> means that the <magnetic field> and screening <electric current> occupy a surface layer, rather than disappearing at a mathematical boundary.<Lecture notes on the electrodynamics of metals and superconductors>[§2, pp. 12–13, Eqs. (64)–(74): acceleration, additional magnetic constraint and penetration length]{ref}
This makes <superconductivity> useful for shielding weak <magnetic fields>, and makes its magnetic response an independent test of the phase. The striking result is that the same screened equilibrium can be reached by two different preparation histories. The assumptions matter: <Finite-thickness Meissner screening> can leave an appreciable <magnetic field> throughout a small sample, and the <Limits of Meissner screening> distinguish <quantum vortex> penetration and retained <magnetic flux> from this equilibrium state.
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