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The expulsion of magnetic flux from the interior of a superconducting metal when it is cooled in a magnetic field to below the critical temperature, near absolute zero, at which the transition to superconductivity takes place. It was discovered by Walther Meissner in 1933, when he measured the magnetic field surrounding two adjacent long cylindrical single crystals of tin and observed that at -452.97°F (3.72 K) the Earth's magnetic field was expelled from their interior. This indicated that at the onset of superconductivity they became perfect diamagnets. This discovery showed that the transition to superconductivity is reversible, and that the laws of thermodynamics apply to it. The Meissner effect forms one of the cornerstones in the understanding of superconductivity, and its discovery led F. London and H. London to develop their phenomenological electrodynamics of superconductivity. See Diamagnetism, Thermodynamic principles
The magnetic field is actually not completely expelled, but penetrates a very thin surface layer where currents flow, screening the interior from the magnetic field.
The Meissner effect is subject to limitations. Full diamagnetism is not observed in polycrystalline samples, and the effect is not observed in impure samples or samples with certain geometrics, such as a round flat disk, with the magnetic field parallel to the axis of rotation. See Superconductivity
the complete expulsion of a magnetic field from a metal conductor when it becomes superconductive (when the intensity of the applied magnetic field is less than the critical value Hc). The Meissner effect was first observed in 1933 by the German physicists W. Meissner and R. Oxenfeld. Partial “freezing” of the magnetic field in the superconductor—that is, incompleteness of the Meissner effect—is observed in insufficiently pure metals and especially in alloys.