The Meissner effect is the expulsion of a magnetic field from a superconductor during its transition to the superconducting state. Walter Meissner and Robert Ochsenfeld discovered the phenomenon in 1933 by measuring the magnetic field distribution outside superconducting tin and lead samples.[1] The samples, in the presence of an applied magnetic field, were cooled below what is called their superconducting transition temperature. Below the transition temperature the samples cancelled all magnetic field inside, which means they became perfectly diamagnetic. They detected this effect only indirectly; because the magnetic flux is conserved by a superconductor, when the interior field decreased the exterior field increased. The experiment demonstrated for the first time that superconductors were more than just perfect conductors and provided a uniquely defining property of the superconducting state.
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In a weak applied field, a superconductor "expels" all magnetic flux. It does this by setting up electric currents near its surface. It is the magnetic field of these surface currents that cancels out the applied magnetic field within the bulk of the superconductor. However, near the surface, within a distance called the London penetration depth, the magnetic field is not completely cancelled; this region also contains the electric currents whose field cancels the applied magnetic field within the bulk. Each superconducting material has its own characteristic penetration depth. Because the field expulsion, or cancellation, does not change with time, the currents producing this effect (called persistent currents) do not decay with time. Therefore the conductivity can be thought of as infinite: a superconductor. Note that a perfect conductor will prevent any change to magnetic flux passing through its surface. This can be explained as ordinary electromagnetic induction and should be distinguished from the Meissner effect. The Meissner effect is the ejection of any magnetic field which occurs during the transition to the superconducting state. Its explanation is more complex and was first given in the London equations by the brothers Fritz and Heinz London.
Superconductors in the Meissner state exhibit perfect
diamagnetism, or superdiamagnetism, meaning that the
total magnetic field B=0 within them. This means that their magnetic susceptibility, 
= −1. Diamagnetism
is defined as the generation of a spontaneous magnetization of a
material which directly opposes the direction of an applied field.
However, the fundamental origins of the diamagnetism in
superconductors and normal materials are very different. In
superconductors the diamagnetism arises from the persistent
screening currents which flow to oppose the applied field; in
normal materials diamagnetism arises as a direct result of an
orbital rotation of electrons about the nuclei of an atom induced
electromagnetically by the application of an applied field. Very
recently, it has been shown theoretically that the Meissner effect
may exhibit paramagnetism in some layered superconductors but so
far this paramagnetic intrinsic Meissner effect has not been
experimentally observed. Mario Rabinowitz and his colleagues
showed that a virtual violation of the Meissner effect is
possible.
The discovery of the Meissner effect led to the phenomenological theory of superconductivity by F. and H. London in 1935. This theory explained resistanceless transport and the Meissner effect, and allowed the first theoretical predictions for superconductivity to be made. However, this theory only explained experimental observations - it did not allow the microscopic origins of the superconducting properties to be identified. Nevertheless, it became a requirement on all microscopic theories to be able to reproduce this effect. This was done successfully by the BCS theory in 1957. It should be noted, however, that the existing theory of the Meissner effect, which includes the phenomenological London's theory, the microscopic BCS one, as well as the classical electrodynamics, is evidently far from completion. The problem is that the electromotive forces described by Faraday's law of induction are equal to zero in stationary conditions of the Meissner effect, whereas the existing theory does not suggest any other electric forces needed to accelerate the electrons until the steady state supercurrent described by the London equation is achieved. Obviously, this acceleration can not be instantaneous for a macroscopic observer, because it would violate the causality principle. The problem was analysed in [2], where a model of the transient supercurrent is suggested. It is based on Cooper pairs as bosons with zero spin and coincides with the London equation asymptotically. However, it requires some arguable extensions of Maxwell-Lorentz electrodynamics.
 A tin cylinder—in a Dewar flask filled with liquid helium—has been placed between the poles of an electromagnet. The magnetic field is about 80 gauss. |
 T=4.2K, B=80 gauss. Tin is in the normally conducting state. The compass needles indicate that magentic flux permeates the cylinder. |
 The cylinder has been cooled from 4.2K to 1.6K. The current in the electromagnet has been kept constant, but the tin became superconducting at about 3K. Magnetic flux has been expelled from the cylinder (the Meissner effect). |
The Meissner effect of superconductivity serves as an important
paradigm for the generation mechanism of a mass M (i.e. a
reciprocal range, 
where h is Planck constant and c is speed of light)
for a gauge field. In fact, this analogy is an abelian example for the Higgs
mechanism, through which in high-energy
physics the masses of the electroweak gauge
particles, W± and Z are generated. The length
is identical with "London's penetration depth" in the theory of superconductivity.
Before the discovery of high-temperature superconductivity, observation of the Meissner effect was difficult, because the applied fields had to be relatively small (the measurements need to be made far from the phase boundary). But with yttrium barium copper oxide, the effect can be demonstrated using liquid nitrogen. Permanent magnets can be made to levitate.
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The Meissner effect (also known as the Meissner-Ochsenfeld effect) is the expulsion of a magnetic field from a superconductor. Walther Meissner and Robert Ochsenfeld discovered the phenomenon in 1933 by measuring the magnetic field distribution outside tin and lead samples. The samples, in the presence of an applied magnetic field, were cooled below what is called their superconducting transition temperature. Below the transition temperature the samples cancelled all magnetic field inside, which means they became perfectly diamagnetic. They detected this effect only indirectly; because the magnetic flux is conserved by a superconductor, when the interior field decreased the exterior field increased. The experiment demonstrated for the first time that superconductors were more than just perfect conductors and provided a uniquely defining property of the superconducting state.
In a weak applied field, a superconductor "expels" all magnetic
flux. It does this by setting up electric currents near its
surface. It is the magnetic field of these surface currents that
cancels out the applied magnetic field within the bulk of the
superconductor. However, near the surface, within a distance called
the London penetration depth, the magnetic field is not completely
cancelled; this region also contains the electric currents whose
field cancels the applied magnetic field within the bulk. Each
superconducting material has its own characteristic penetration
depth. Because the field expulsion, or cancellation, does not
change with time, the currents producing this effect (called
persistent currents) do not decay with time. Therefore the
conductivity can be thought of as infinite: a superconductor. Note
that field expulsion (or cancellation) implies infinite
conductivity, but that infinite conductivity does not imply field
expulsion, because if a material develops only infinite
conductivity below its transition temperature, that infinite
conductivity will "freeze in" whatever magnetic field was present
as the transition temperature was reached. The theory for field
expulsion was given in the London equations by the brothers Fritz
and Heinz London.
http://en.wikipedia.org/wiki/Meissner_effect
Superconductor w:Meissner Effect
For Walther Meißner, German physicist.
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