The Story of Super Conductivity

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The Story of Super Conductivity

A superconductor is an element, inter-metallic alloy or compound that will conduct electricity without resistance below a certain temperature. Resistance is undesirable as it produces losses in energy flowing through the material. Once set in motion electrical current will flow forever in a loop of super conducting making it the closest thing to perpetual motion in nature. In essence superconductivity is a phenomenon displayed by certain conductors that demonstrate no resistance to the flow of an electric current at a certain temperature.

The Dutch physicist, Heike Kammerlingh Onnes, first discovered superconductivity in 1911. Onnes' career was dedicated to exploring extremely cold refrigeration. On July 10, 1908, he successfully liquefied helium by cooling it to 452 degrees below zero Fahrenheit (4 Kelvin or 4 K). Liquid helium enabled him to cool other materials closer to absolute zero (0 Kelvin), the coldest temperature imaginable. Absolute zero is the temperature at which the energy of material becomes as small as possible (however this is only a theoretical temperature and may never be achieved).

In 1911, Onnes began to investigate the electrical properties of metals in extremely cold temperatures. Onnes as well as many scientists had known for many years that the resistance of metals fell when cooled, but it was not known what limiting value the resistance would approach, if the temperature were reduced to very close to 0 K. William Kelvin, along with many other scientists believed that electrons flowing through a conductor would come to a complete halt as the temperature approached absolute zero. However other scientists, including Onnes, felt that a cold wire's resistance would disperse. This suggested that there would be a steady decrease in electrical resistance and better conduction of electricity. At some very low temperature point, scientists felt that there would be a leveling off as the resistance reached a minimum value allowing the current to flow with little or no resistance. Onnes decided to investigate this and passed a current through a very pure mercury wire and measured its resistance as he steadily lowered the temperature. To his surprise there was no leveling off of resistance, let alone the stopping of electrons as suggested by Kelvin. At 4.2 K the resistance suddenly vanished. Current was flowing through the mercury wire and nothing was stopping it, the resistance was zero. Figure (1) is a graph showing resistance against temperature in mercury wire just as Onnes measured. Onnes said, "Mercury has passed into a new state, which on account of its extraordinary electrical properties may be called the superconductive state". The experiment left no doubt about the disappearance of the resistance of a mercury wire. Onnes called this newly discovered state, Superconductivity.
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Onnes recognized the importance of his discovery an electrical conductor with no resistance could carry current any distance with no loss. In one of Onnes experiments he started a current flowing through a loop of lead wire cooled to 4 K. A year later the current was still flowing without a significant loss in current. Onnes found that the superconductor exhibited what he called persistent currents, electric currents that continued to flow without an electric potential driving them. Onnes discovered superconductivity, and was awarded the Nobel Prize in 1913.

By 1933 Walther Meissner and R. Ochsenfeld discovered ...

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