Meissnerilmiö

Meissnerilmiö, also known as the Meissner effect, is a phenomenon observed in superconductors. When a material becomes superconducting, it expels all magnetic field lines from its interior. This expulsion is complete, meaning the magnetic field inside the superconductor becomes zero, regardless of the strength of the external magnetic field. This is distinct from a perfect conductor, which would only prevent changes in magnetic flux.

The Meissner effect was discovered in 1933 by Walther Meissner and Robert Ochsenfeld. They observed that a superconducting tin sample, when cooled below its critical temperature in the presence of a magnetic field, pushed the magnetic field lines out. This expulsion is a defining characteristic of superconductivity.

The strength of the expelled magnetic field is limited. If the external magnetic field is too strong, it can destroy the superconducting state, a phenomenon known as the critical magnetic field. For Type I superconductors, the expulsion is complete up to this critical field. Type II superconductors exhibit a more complex behavior, allowing some magnetic flux to penetrate their interior in a quantized form above a lower critical field, but still expelling fields completely below this point. The Meissner effect is a macroscopic quantum mechanical phenomenon and is crucial for understanding the properties and applications of superconductors.