![]() ![]() There are many criteria by which superconductors are classified. Main article: Superconductor classification The cheaply available coolant liquid nitrogen boils at 77 K (−196 ☌) and thus the existence of superconductivity at higher temperatures than this facilitates many experiments and applications that are less practical at lower temperatures. Such a high transition temperature is theoretically impossible for a conventional superconductor, leading the materials to be termed high-temperature superconductors. In 1986, it was discovered that some cuprate- perovskite ceramic materials have a critical temperature above 90 K (−183 ☌). The occurrence of the Meissner effect indicates that superconductivity cannot be understood simply as the idealization of perfect conductivity in classical physics. It is characterized by the Meissner effect, the complete ejection of magnetic field lines from the interior of the superconductor during its transitions into the superconducting state. Like ferromagnetism and atomic spectral lines, superconductivity is a phenomenon which can only be explained by quantum mechanics. The superconductivity phenomenon was discovered in 1911 by Dutch physicist Heike Kamerlingh Onnes. Meissner effect in a high-temperature superconductor (black pellet) with a NdFeB magnet (metallic) A high-temperature superconductor levitating above a magnet This current effectively forms an electromagnet that repels the magnet. Persistent electric current flows on the surface of the superconductor, acting to exclude the magnetic field of the magnet ( Faraday's law of induction). A magnet levitating above a high-temperature superconductor, cooled with liquid nitrogen. An electric current through a loop of superconducting wire can persist indefinitely with no power source. Unlike an ordinary metallic conductor, whose resistance decreases gradually as its temperature is lowered, even down to near absolute zero, a superconductor has a characteristic critical temperature below which the resistance drops abruptly to zero. Any material exhibiting these properties is a superconductor. Superconductivity is a set of physical properties observed in certain materials where electrical resistance vanishes and magnetic fields are expelled from the material. ![]() Electrical conductivity with exactly zero resistance ![]()
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