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A magnet levitating above a high-temperature superconductor, cooled with liquid nitrogen. Persistent electric current flows on the surface of the superconductor. This excludes the magnetic field of the magnet (Faraday's law of induction). In effect, the current forms an electromagnet that repels the magnet

A superconductor is a substance that conducts electricity without resistance when below a critical temperature. At this temperature electrons can move freely through the lattice. High magnetic fields destroy superconductivity and restore the normal conducting state.

We expect a magnet moving by a conductor to induce a currents in the conductor by electromagnetic induction. But, a superconductor expels magnetic fields by inducing surface currents.  The Meissner effect is demonstrated by levitating a superconductor over magnets or vice versa.

History of superconductors[change | change source]

1911 superconductivity discovered by Heike Kamerlingh Onnes
1933 the Meissner effect discovered by Walter Meissner and Robert Ochsenfeld
1957 theoretical explanation for superconductivity put forward by John Bardeen, Leon Cooper, and John Schrieffer (BCS theory)
1962 the tunneling of superconducting Cooper pairs through insulating barrier predicted
1986 A ceramic superconductor was discovered by Alex Müller and Georg Bednorz. Ceramics are normally insulators. A lanthanum, barium, copper and oxygen compound with a critical temperature of 30K. Opened up the possibilities for new superconductors.

Applications[change | change source]

  • Superconducting quantum interference device (SQUID)
  • Particle accelerators
  • Small particle accelerators in health
  • Levitating trains
  • Nuclear fusion
  • MRI Scanner