Superconductivity

the resistance of metals varies with their temperatures. when they get hot, their resistance increases. when they cool, their resistance fall. the resistance of some metals and alloys steadily decreases as their temperatures is lowered, then falls suddenly to negligible value at temperatures a few degrees above absolute zero. in other words, these materials have almost no resistance to an electric current at very low temperatures. they become almost perfect conductors. this is called superconductivity. it occurs with certain materials, for example lead, and only at very low temperatures.

The practical applications of superconductivity are limited because of the very low temperatures required. A number of uses, however, have been proposed. If a current is induced by a magnetic field in a ring of superconducting material, it will continue to circulate when the magnetic field is removed. In theory, this could be made use of in the memory cells of computers. Memory cells made of superconducting materials could store information indefinitely. Because of the zero resistance of the cells, the information could be retrieved very quickly, as fast as 10-8 seconds.

Ninety percent of the totals losses in a modern transformer are due to the resistance of the windings. Transformer could be made with the windings cooled to the very low temperatures at which superconductivity occurs. The resistance of the winding would be zero and the transformer would be almost ideal. Similarly a 100% efficient electric motor has been proposed using the magnetic field of superconducting coils.