How Was It Discovered? 129
However, there was a problem. No method existed to cool anything anywhere near
-269°C. Luckily, Onnes was the physics department chair at the University of Leyden, and
that department came equipped with a well-funded physics lab that Onnes could use.
In 1907 Onnes invented thermometers that could measure temperatures as extreme as
absolute zero. In 1908 he discovered a way to cool the gas helium so cold that it turned into a
liquid. He was able to continue to chill the super-cold liquid until, late that year, he chilled
liquid helium to 0.9°K—less than one degree above absolute zero! Onnes realized that he
could use this liquid helium to chill other materials to near 0°K to measure their electrical
resistance.
By 1911 Onnes had developed canisters capable of holding and storing his super-cold
liquid helium and had set up a small production line. He began his electrical studies by chilling first platinum and then gold to near absolute zero. However, the electrical currents he
measured were erratic, his results inconclusive.
Onnes decided to switch to liquid mercury. He filled a U-shaped tube with mercury
and attached wires to each end of the U. The wires were attached to a meter to measure electrical resistance. He used liquid helium at 0.9°K to cool the mercury.
As the temperature dipped below 40°K (-229°C) electrical resistance began to drop. It
dropped steadily as the temperature dipped below 20°K. And then, at 4.19°K resistance
abruptly disappeared. It fell to zero.
Onnes repeated the experiment many times over the next few months and always got
the same result. Below 4.19°K, there was no resistance to the flow of electricity. An electric
current would flow unimpeded for ever! He called it superconductivity.
Onnes had discovered superconductivity, but he could not theoretically explain it. He
only suspected that it had something to do with the (then) recently discovered Quantum
Theory. It was not until 1951 that John Bardeen developed a mathematical theory to explain
superconductivity.
A search began to find ways to create superconductivity at higher (more practically
reached) temperatures. The current record (using—unfortunately—toxic ceramic compounds made with mercury and copper) is 138°K (-131°C). Once a way is found to create
superconductivity at warmer temperatures, the value of Onnes’s discovery will be
unlimited.
Fun Facts: AT CERN, the European high-energy physics research lab,
scientists used a one-time jolt of electricity to start an electrical current
flowing through a superconductor circuit. That electrical current
ran—with no additional voltage input—for five years with no loss of
power. In common house wires, an electrical current would stop within a
few milliseconds once the voltage is removed because of the resistance
of electrical wires.