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I INTRODUCTION Superconductors An element or compound that will conduct electricity without resistance below acertain temperature is a superconductor This applies only to direct current electricity andto limited amounts of current Once it is set in motion the current will flow forever in aclosed loop of superconducting material Superconductivity is manifested only below acertain critical temperature and a critical magnetic field which vary with the material usedBefore 1986 the highest critical temperature was 232 Kelvin which would be -4176FTemperatures this low were achieved by use of liquid helium an expensive inefficientcoolant Ultra low-temperature operation places a severe constraint on the overallefficiency of a superconducting machineII History Superconductivity was first discovered in 1911 by the Dutch physicist HeikeKamerlingh Onnes who observed no electrical resistance in mercury below 42 K or-4518 F The phenomenon was better understood only after strong diamagnetism wasdetected in a superconductor by Karl W Meissner and R Ochsenfeld of Germany in 1933The basic physics of superconductivity however was not realized until 1957 when theAmerican physicists John Bardeen Leon N Cooper and John R Schrieffer advanced thenow celebrated BCS theory for which the three were awarded the 1972 Nobel Prize inphysics The theory describes superconductivity as a quantum phenomenon in which theconduction electrons move in pairs and show no electrical resistance In 1962 the Britishphysicist Brian D Josephson examined the quantum nature of superconductivity andproposed the existence of oscillations in the electric current flowing through twosuperconductors separated by a thin insulating layer in a magnetic or electric field Theeffect known as the Josephson effect subsequently was confirmed by experimentsIII APPLICATIONS Because of the lack of resistance of superconductors they have been used tomake electromagnets that generate large magnetic fields with no energy lossSuperconducting magnets have been used in diagnostic medical equipment studies ofmaterials and in the construction of powerful particle accelerators By using the quantumeffects of superconductivity devices have been developed that measure electric currentvoltage and magnetic field with unprecedented
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