The Meissner effect, named after German physicist Walther Meissner, is a fundamental phenomenon in superconductivity. It refers to the expulsion of a magnetic field from a superconductor when it transitions into the superconducting state. This effect is one of the defining characteristics of superconductors and is a direct consequence of the perfect diamagnetism of these materials.
In simple terms, the Meissner effect is the complete expulsion of magnetic field lines from the interior of a superconductor as it transitions from the normal to the superconducting state. This is in stark contrast to the behavior of ordinary conductors, which allow magnetic fields to penetrate their interiors.
The Meissner effect is a manifestation of the superconductor's perfect diamagnetism. Diamagnetism is a property of all materials and is the tendency to oppose an applied magnetic field, but in superconductors, this opposition is perfect and complete.
The Meissner effect is crucial in establishing and maintaining the superconducting state. When a superconductor is cooled below its critical temperature in the presence of a magnetic field, it expels all internal magnetic fields to maintain zero electrical resistance. This expulsion is not a passive process but an active one, involving the generation of supercurrents on the surface of the superconductor that exactly cancel the applied magnetic field.
The Meissner effect was first discovered in 1933 by Meissner and Robert Ochsenfeld. They observed that a superconducting tin sample expelled an applied magnetic field upon cooling below its critical temperature. This discovery was a crucial step in the understanding of superconductivity, as it showed that superconductors were more than just perfect conductors.
The Meissner effect can be directly observed in a simple experiment involving a superconductor and a magnet. When a superconductor is cooled below its critical temperature in the presence of a magnet, it will levitate the magnet by expelling the magnetic field. This levitation is a dramatic demonstration of the Meissner effect.
The Meissner effect has several practical applications. It is the principle behind the operation of superconducting magnets, which are used in a variety of technologies, including magnetic resonance imaging (MRI) machines and particle accelerators. The Meissner effect is also crucial in the operation of superconducting magnetic energy storage (SMES) systems, which store energy in the magnetic field created by a superconducting coil.
In conclusion, the Meissner effect is a fundamental aspect of superconductivity. It is a direct manifestation of the perfect diamagnetism of superconductors and is crucial in maintaining the superconducting state. The Meissner effect has been extensively studied and has several important applications in modern technology.