In 1911, Dutch physicist Heike Kammerlingh Onnes discovered a phenomenon called superconductivity when he realised that if mercury is cooled to 4.1 Kelvin (or -269 Celsius, near absolute zero), it loses all electrical resistance. Many other metals exhibit this property too, but only below a “critical temperature”, the value of which is inversely proportional to the square root of the material’s atomic mass. Zero resistance has been demonstrated by sending currents through superconducting wires—the current loses no energy to resistance. If the wire is then bent into a loop, the current will keep going for years with no measurable reduction, while in ordinary material, resistance would make the current rapidly decay. One awesome property of a superconductor is that it excludes magnetic fields, causing a phenomenon called the Meissner effect. Basically, if a magnet is brought near a superconductor, it encounters a repulsive force because the superconductor completely expels the magnetic field. Levitate a magnet above a superconductor, and it’ll hover there forever. The uses of this are widespread—from proton accelerators to high-speed magnetically-levitated trains to the more common superconducting magnets for MRI. Superconductivity has inspired dreams of no-loss electrical transmission, but the future of research is to find materials that become superconductive at room temperature.