Kamerlingh Onnes (Nerderland, 1853-1926, Nobel in 1913 ) made two significant discoveries: helium liquefaction, and superconductivity, which paved the way to hitherto unexplored fields of research, with extremely numerous new concepts and applications. He was the student of another great physicist, J.D. van der Waals, and in 1882, he became chairman of the physics department at the University of Leiden.
He only had one goal: managing to liquefy helium gas to break the coldest temperature record. In order to do that, he created the first actual industrial laboratory. In 1908, Kamerlingh Onnes and his head mechanic Flim finally managed to reach a temperature of one degree above absolute zero (the temperature where everything becomes absolutely motionless) (article about their discovery ). To achieve that, they cooled and compressed helium gas thanks to liquid hydrogen, before suddenly expanding it, hence cooling it even more until it liquefied. This is how they paved the way for a new field of research: the study of very low temperatures.
Right after his discovery, Kamerlingh Onnes tried to cool numerous materials and study their properties. There was one simple question he was asking himself in particular: what happens to the electric resistance of a metal when its temperature gets close to absolute zero? Faithful to his motto “Door meten tot weten”, “knowledge through measurement”, Kamerlingh Onnes, helped by his assistant Gilles Holst, measured the electric resistance of mercury cooled by liquid helium on April 8th, 1911. That day, he wrote in his experiment notebook: “Temperature successfully measured. Mercury almost at zero”. He had just made one of the most astonishing experimental discoveries of the 20th century: resistance was not increasing; it was on the contrary dramatically dropping to zero! Kamerlingh Onnes had just proved the existence of superconductivity. (Article about their discovery). The same day, he observed that helium stops boiling two degrees above absolute zero, but he did not dwell on it. He did not know it, but he was the first person to observe helium superfluidity !
A movie which explains the way superconductivity was discovered on 8 april 1911, with an interview of D. Van Delft, from museum Borhaave (Leyden)