On April 8, 1911, Dutch physicist and Nobel Laureate Heike Kamerlingh Onnes found that at a temperature of only 4.2 K (-269° C) the resistance in a solid mercury wire immersed in liquid helium suddenly vanished. Kamerlingh Onnes discovered superconductivity.
Through measurement to knowledge – “Through measurement to knowledge”
Motto of Kamerlingh Onnes’ Laboratory
Born in Groningen, Netherlands, Kamerlingh also attended the city’s university and studied under the famous Robert Bunsen  and Gustav Kirchhoff  at the University of Heidelberg in the early 1870s. In 1871, Kamerlingh Onnes‘ talents for solving scientific problems was already apparent, since he was awarded a Gold Medal for a competition sponsored by the Natural Sciences Faculty of the University of Utrecht, followed the next year by a Silver Medal at the University of Groningen. Kamerlingh’s dissertation was titled “Nieuwe bewijzen voor de aswenteling der aarde” (New proofs of the rotation of the earth) and he became assistant to Johannes Bosscha, Dutch physicist and director of the TU Delft.
Starting in 1882, Kamerlingh Onnes served as professor of experimental physics at the University of Leiden, where he stayed until 1923. Onnes founded a very large cryogenics laboratory and invited other researchers to the location, which made him highly regarded in the scientific community. The laboratory is now known as Kamerlingh Onnes Laboratory. About four years after founding the famous laboratory, Onnes managed to liquefy helium through multiple precooling stages and the Hampson-Linde cycle based on the Joule-Thomson effect. Doing this, Onned was able to lower the temperature to the boiling point of helium, which is about -269°C. Onnes reduced the pressure of the liquid helium and thus achieved temperatures near 1.5 K, which were the coldest temperatures achieved on earth at the time.
In 1911 Kamerlingh Onnes managed to measure the electrical conductivity of pure metals at very low temperatures. He believed, that a conductor’s electrical resistance would steadily decrease and drop to nil. In April of the same year, he found out that at 4.2 K the resistance in a solid mercury wire immersed in liquid helium suddenly vanished and almost immediately realized the significance of his amazing discovery. He then reported that “Mercury has passed into a new state, which on account of its extraordinary electrical properties may be called the superconductive state“. Further articles on the phenomenon were published by Onnes and he referred to it as “supraconductivity”. Only later on, the term was changed to “superconductivity“. The instruments used for these experiments are now demonstrated at the Boerhaave Museum in Leiden.
Onnes‘ reputation grew drastically and he received the Nobel Prize in Physics in 1913 for “his investigations on the properties of matter at low temperatures which led, inter alia, to the production of liquid helium“.
At yovisto academic video search , you may be interested in a short introduction to Superconductivity.
References and Further Reading:
-  Kamerlingh Onnes at the Nobel Peace Prize Website
-  Leiden University historical web site
-  Through Measurement to Knowledge: The Inaugural Lecture of Heike Kamerlingh Onnes
-  Robert Wilhelm Bunsen and the Bunsen Burner, SciHI Blog
-  Gustav Kirchhoff and the Fundamentals of Electrical Circuits, SciHi Blog
-  Heike Kamerling Onnes at Wikidata
-  Timeline for Heike Kammerlingh Onnes, via Wikidata