Superconductors carry electric current with zero resistance
In 1911 chilled mercury lost all electrical resistance. Here is how physicists know it is truly zero, and why room temperature still eludes them.
Superconductors carry electric current with zero electrical resistance once cooled below a critical temperature: in 1911, Dutch physicist Heike Kamerlingh Onnes found that the resistance of purified mercury dropped suddenly to nothing at about 4.2 kelvin (−269 °C).
He was settling an argument, not hunting for a new state of matter. Onnes had liquefied helium in 1908, and used that cold to test Lord Kelvin’s view that metals would stop conducting electricity altogether near absolute zero. Instead the resistance vanished. Onnes first called the effect “supraconductivity”; the name later settled as superconductivity.
How do we know it is really zero?
No meter can read exactly zero, so physicists set a current circulating in a closed superconducting ring and watch for it to fade. In an ordinary metal ring it dies almost at once. Currents in lead rings have been kept running for several years, with decay so slight it implies a time constant of over a billion years. The explanation came in 1957, when Bardeen, Cooper and Schrieffer showed that at low temperatures electrons couple into pairs that flow through the metal without resistance.
The prize is practical. About 6% of all electricity distributed in the United States is lost in transmission and distribution, and lossless superconducting lines, coils and maglev magnets have been dreamed of for a century. The catch is the cold.
In 1986, Georg Bednorz and K. Alex Müller at IBM found copper-based ceramics that superconduct at 35 K, and other teams soon raised such materials high enough to be cooled with cheap liquid nitrogen. Hydrogen-rich superconductors work warmer still, but only at more than a million times Earth’s atmospheric pressure.
A superconductor at room temperature and ordinary pressure remains unfound. The most recent viral claim, the Korean compound LK-99 in July 2023, collapsed within weeks: after dozens of replication attempts, specialists concluded it is not a room-temperature superconductor.
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