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When an electric current passes through any material, some part of that current is lost to resistance. A material with little resistance to the flow of electricity is called a conductor, and these materials are useful for building electric devices. Many conductors, when cooled to temperatures near absolute zero, lose all resistance to current. A material with no resistance to the flow of electricity is a superconductor. Recent developments hint that some unusual materials may be superconductive even at room temperatures.
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A superconductor is a pure metal that at low temperatures has negligible resistance to the flow of electric current. Each material has its own critical temperature, Tc, above which it is a normal conductor. When a current is established, it persists almost indefinitely. Magnetic fields can destroy the superconductivity, their strength depending on how far below the critical temperature the material is.

Generally, Tc has been perovskite structures were discovered, where Tc ~ 90 K.

See Meissner effect.

A superconductor works the way it does because of the formation of electrons.

Normally, electrons move one at a time through the conductor while having their energy run down from electrical resistance. When an electron moves through two positive ions, the ions are attracted towards the electron and so move closer to each other. A positive charge is formed from the two ions who then attract an electron. The original is effectively a pair as it has another electron trailing it.

This is called a Cooper Pair. All the electrons in the superconductor will eventually combine into these Cooper Pairs, which has the net result of no electron being able to slow down as it is locked into another. All the electrons in the superconducting material will be able to move across it smoothly without losing any energy in the process.

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Just how much energy is saved in a superconductor? Half of the electricity in North America could be sent to Japan on a superconducting wire as thick as a basketball.

An interesting property of superconductors it their impenetrability to magnetic fields. This is used as a test for materials making the phase change to superconductivity - all magnetic fields within the material will be expelled. This makes a superconductor act like a magnetic mirror - you can even get a magnet to float above a superconductor as if it was being repelled by its "image".