The Meissner effect (or Meissner-Ochsenfeld effect) is the effect by which a weak magnetic field decays rapidly to zero in the interior of a superconductor. The distance to which the field is active is known as the London penetration depth. This active exclusion of magnetic fields is distinct from perfect diamagnetism. It is seen that the magnetic field will be zero inside the material in the superconducting state regardless of what it was before the material became superconducting. It was discovered by Walther Meißner and Robert Ochsenfeld in 1933. The Meissner effect is one of the defining features of superconductivity, and its discovery served to establish that the onset of superconductivity is a phase transition.

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