For many years, scientists had known that the electrical resistance of a metal is proportional to its temperature. That is to say that as you cool a metal down, so its resistance to an electric current drops. However, until helium was liquefied in 1908, there was no means of reaching the very low temperatures just above absolute zero.
One day in 1911, the Dutch physicist Heike Kammerlingh Onnes began cooling some mercury with liquid helium. As the temperature lowered, so too did the metalís resistance, but at 4.2 K, something very strange happened. The resistance of the metal completely disappeared and Onnes had discovered superconductivity. He was awarded the Nobel Prize in 1913.
Despite the broad range of compounds (both inorganic and organic) that are known to superconduct at very low temperatures, often below the boiling point of liquid helium, the highest superconducting transition temperature recorded before 1986 was 23K for the alloy Nb3Ge.
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