The invention of a technique to liquefy helium by Heike Kamerlingh Onnes in 1908 provided scientists with a means of reaching an entirely new range of low temperatures in the vicinity of absolute zero. Helium liquefies at 4.2 K and using this newly accessible range of temperatures to investigate the electrical properties of materials, Onnes found an abrupt drop in electrical resistance where the resistance of the mercury wire he was examining became so low that he could not measure it. This was the first time that anyone had encountered the phenomenon of perfect conduction or “superconductivity”. In addition, the same class of material was later found to expel magnetic fields. The combination of perfect conduction and perfect magnetic field expulsion is what defines a superconductor.
The often weird and strange world of quantum mechanics is generally considered to be confined to the atomic level, but with the introduction of superconductivity we actually find a state of matter which exhibits some of the bizarre quantum properties at a macroscopic level. This is what makes superconductivity such an exciting area of study and leads to some new and strange effects.