In 1911, the Dutch Physicist, Kamerlingh Onnes established experimentally that certain metals and alloys lose their resistance at a very low temperature. He observed that when purified mercury is cooled, its resistivity disappears abruptly at 4.2 K. Above this temperature, the resistivity is small, but finite. This new phenomenon observed by Onnes is called superconductivity. The phenomenon is exhibited by a number of elements and many alloys.
