At very low temperature most of the metals and alloys allows the current to pass through it freely without resistance. This phenomenon is called Superconductivity.
It was discovered in 1911 by Kammerlingli Onnes. He was experimenting with mercury sample at low temperature and studies the resistance of mercury. The Resistivity of the mercury was decreasing normally when the temperature was decreases. But at a particular temperature TC = 4.15 K the resistivity suddenly dropped to zero. This property is called Superconductivity. The superconducting sate the conductional electrons in the metal forms a loosed pair that persistence the electric current.
Superconducting property occurs in many metals, alloys, metalloids and doped semiconductors. Some material became super conducting only under high pressure. Eg. Silicon TC = 8.3 K; pressure = 165 kilo bar. Even trace quantities of foreign bodies if magnetic can lower the transition temperature very much. Decay time of the superconductor is not less than 100,000 year.
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