The electrical resistivity of a metal arises from the interactions of the conduction electrons with impurities, defects and the vibrating ions of the lattice. As the temperature is lowered, the amplitudes of the lattice vibrations diminish, so one would expect the resistivity also to decrease gradually toward a small, but finite, value determined by the impurities and defects. This behavior is manifested by many materials. In 1911 H. Kamerlingh Onnes discovered that as the temperature of mercury was reduced, its resistance suddenly dropped to an extremely small value at 4.15 K. The metal had made a transition to a new superconducting state. The resistivity of a superconductor is at least a factor of 10-12 less than that for an ordinary conductor. We can usually take it to be zero. In superconductors electrical energy can be transported without resistive losses, provided the temperature is maintained below the critical temperature, Tc. ‘High temperature superconductors’ are such that remains superconductors in relatively high temperatures (for example 77K). In addition to their obvious use in electromagnets, there are other applications of superconductors. The resistive losses in transmission lines amount to about 10 % of the power supplied. This heat dissipation would be eliminated by superconducting lines. Persistent currents around a hole in a superconductor may be used as a memory device and so on. In 1933, W. Meissner and R. Ochsenfeld placed a sample of lead in a weak magnetic field and started to cool it. They found that as the material made the transition to the superconducting state, the flux was expelled. This is called the Meissner-Ochsenfeld effect. It shows that a superconductor is characterized by more than just perfect conductivity; it also displays perfect diamagnetism. There exist two types of superconductors:
Type I superconductor has one critical field, above which superconducting regions diminish in size and finally disappear in critical...
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