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Meissner effect
The Meissner effect is the total exclusion of any magnetic flux from the interior of a superconductor. It was discovered by Walter Meissner[?] and Robert Ochsenfeld[?] in 1933 and it is often referred to as perfect diamagnetism or the Meissner-Ochsenfeld effect.
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.
The exclusion of magnetic flux is brought about by
electrical "screening currents" that flow at the surface of
the superconducting metal and which generate a magnetic field that exactly cancels the externally applied field
inside the superconductor. These screening currents are
generated whenever a superconducting metal
is brought inside a magnetic field. This may be
understood from the fact that a superconductor has
zero electrical resistance (so the "eddy currents"
induced by motion of the metal inside a magnetic field
will not decay). However, the screening currents
also appear in a situation where an initially normal conducting metal
is placed inside a magnetic field, as soon as the metal
is cooled below the transition temperature
(such that it becomes superconducting). This expulsion
of magnetic field on cooling down the metal cannot
be explained any more by merely assuming zero resistance.
It shows that the superconducting state does not
depend on the history of preparation
(only on the present values
of temperature, pressure and magnetic field), and
therefore is a true thermodynamic state.
Superconducting magnetic levitation is due to the Meissner effect (which repels a permanent magnet) and flux pinning[?], which stops the magnet from sliding away.
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