A superconductor is an element or compound that will conduct electricity without resistance below a cretin temperature. The phenomenon was discovered in 1911 by Kamerlingh Onnes, who found that the resistance of mercury dropped suddenly to zero at a temperature of about 4.2°K. For the next 75 years there followed a rather steady string of announcements of new materials that become superconducting near absolute zero. A major breakthrough occurred in 1986 when Karl Alexander Müller and J. Georg Bednorz announced that they had discovered a new class of copper-oxide materials that become superconducting at temperatures exceeding 70°K. The work of Müller and Bednorz, which earned them the Nobel Prize in Physics in 1987, precipitated a host of discoveries of other high-temperature superconductors that exhibit lossless electrical flow at temperatures up to 125°K. Classical superconductivity (superconductivity at temperatures near absolute zero) is displayed by some metals, including zinc, magnesium, lead, gray tin, aluminum, mercury, and cadmium. Other metals, such as molybdenum, may exhibit superconductivity after high purification. Alloys (e.g., two parts of gold to one part of bismuth) and such compounds as tungsten carbide and lead sulfide may also be superconductors. Thin films of normal metals and superconductors that are brought into contact can form superconductive electronic devices, which replace transistors in some applications. An interesting aspect of the phenomenon is the continued flow of current in a superconducting circuit after the source of current has been shut off: for example, if a lead ring is immersed in liquid helium, an electric current that is induced magnetically will continue to flow after the removal of the magnetic field. Powerful electromagnets, which, once energized, retain magnetism virtually indefinitely, have been developed using several superconductors. The 1972 Nobel Prize in Physics was awarded to J. Bardeen, L. Cooper, and S. Schrieffer for their theory (known as the BCS theory) of classical superconductors. This quantum mechanical theory proposes that at very low temperatures electrons in an electric current move in pairs. Such pairing enables them to move through a crystal lattice without having their motion disrupted by collisions with the lattice. Several theories of high-temperature superconductors have been proposed, but none has been experimentally confirmed.
Magnetic levitation is an application where superconductors perform extremely well. Transport vehicles such as trains can be made to "float" on strong superconducting magnets, virtually eliminating friction between the train and its tracks. Not only would conventional electromagnets waste much of the electrical energy as heat, they would have to be physically much larger than superconducting magnets. A landmark for the commercial use of MAGLEV technology occurred in 1990 when it gained the status of a nationally funded project in Japan. The Minister of Transport authorized construction of the Yamanashi Maglev Test Line, which opened on April 3, 1997. Two years later on April 14, 1999, the MLX01 test vehicle attained an incredible speed of 343 miles per hour.
Although the technology has now been proven, the wider use of MAGLEV vehicles has been constrained by political and environmental concerns. The world\'s only MAGLEV train to be adopted into commercial service, a shuttle in Birmingham, England, shut down in 1997 after operating for 11 years. Meanwhile, the U.S. government has earmarked nearly a billion dollars to build a MAGLEV train at one of seven proposed sites. And Germany\'s commercial MAGLEV is expected to become operational in 2006.
An area where superconductors can perform a life-saving function is in the field of biomagnetism. Doctors need a non-invasive means of determining what\'s going on inside the human body. By impinging a strong superconductor-derived magnetic field into the body, hydrogen atoms that exist in the body\'s water and fat molecules are forced to accept energy from the magnetic field. They then release this energy at a frequency that can be detected and displayed graphically by a computer. Magnetic Resonance Imaging (MRI) was actually discovered in the mid 1940\'s. But, has only recently become an indispensable medical tool with the development of powerful computers to quickly process the large volume of data that is generated.