For just a while, Leiden was the coldest point on earth. The Klondike of the West, with mercury colder than the South Pole. It was the result of an experiment by Heike Kamerlingh Onnes, which he repeated for a second time exactly 111 years ago today. And this time, it led to the discovery of superconductivity. At an extremely low temperature – close to absolute zero – the electrical resistance of pure metals disappears completely. Heike received the Nobel Prize in Physics for his discovery. And we make use of his findings. Think of liquid helium, for example!