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Resistance in a superconductor
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In a superconductor : discovery

Why and how did Kamerlingh Onnes discover superconductivity?

At the beginning of the 20th century, Kamerlingh Onnes wanted to measure and understand the properties of metals at very low temperatures. Until then, we only knew that the resistance in a metal progressively drops when the temperature is decreased.

But what would happen if we cooled the metal to absolute zero? Would the resistance continue to slowly decrease, would it reach an absolute minimum, or would it approach an infinite value? The first measurements showed a saturation to a minimum of resistance, but this limit seemed to depend on the purity of the metal. This is the reason why Onnes decided to measure a metal he knew how to purify: mercury.

K. Onnes measures a sudden drop of the resistance of mercury versus temperature : he just discovered superconductivity.<br/>Credits: Museum Boerhaave, LeidenOn April 8th, 1911, Onnes discovered to his own amazement that below 4.2 K (about -269°C), the resistance of mercury suddenly dropped to zero!

K. Onnes logbook on the day of the discovery, 8 april 1911, which mentions 'mercury almost zero'.<br/>Credits: Museum Boerhaave, LeidenThis experiment can be repeated showing a very sudden drop which is completely unexpected and inexplicable. This happens as if the metal electrons suddenly started to move forward with nothing to slow them down. K. Onnes called this phenomenon “superconductivity”, a nickname he invented on the day he received the Nobel Prize.

Shortly after, Kamerlingh Onnes decided to conduct another experiment in order to prove that the resistance had actually completely disappeared.  After applying an electric current to a ring made of tin, another superconductor, he observed that the current does not disappear, confirming his first measurements.


CNRSSociété Française de PhysiqueTriangle de la physique
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CNRSSociété Française de PhysiqueTriangle de la physique