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Superconductors

Superconductivity materials lose all resistance and exclude external magnetic fields when they become superconducting. The temperature at which they become superconducting is known as transition or critical temperature Tc. At temperature above Tc the resistance of the material is not zero.

In like manner the magnetic field strength at which a superconducting material loses its superconductivity is the critical field. A useful superconductor has a high critical temperature and field. In January 1987 superconductivity critical temperature of a compound of yttrium, barium, and copper oxide was found to be about 90o K. This is above the temperature of liquid nitrogen, which boils at 77o K. Higher critical temperatures have been reported, but have not been reliably reproduced yet. The new superconducting compounds are brittle, but have been drawn into fine wires. Superconducting materials found so far show promise for electronic devices. Much work remains to be done before they can be used for superconducting transmission lines.
Short experimental superconducting lines have been built and operated, but have not yet been shown to be economical.