Most electrical conductors follow Ohm's law and exhibit some electrical resistance. A superconductor shows a peculiar low-temperature phenomenon of zero electrical resistance. A current in a loop of superconductor will cycle forever.
Superconductivity emerges at extremely low temperatures, near absolute zero (-273°C or -460°F). Even a normal conductive metal decreases its electrical resistance as the temperature drops, but the resistance is never zero. Certain materials, though, abruptly change their physical characteristics when the temperature drops below a critical value.
A superconductor will run a current forever without losing energy or generating resistance heat. A superconductor expels any weak magnetic field in the environment, which will flow around its surface but not in it. This leads to odd visuals, such as a lightweight magnet floating above the superconductor (right). A sufficiently powerful magnetic field, though, will end the superconducting effect.
Superconductivity is a quantum effect, meaning quantum mechanics can explain it, but not classical physics.
Superconductivity was discovered by Dutch physicist Heike Kamerlingh Onnes in 1911, also the person who first liquefied helium. Kamerlingh Onnes discovered that when a solid mercury wire was bathed in liquid helium, the resistance suddenly dropped to zero.
Lead, and then other substances, were discovered to exhibit superconductivity. Commercial applications did not appear until 50 years later, with development of the Josephson junction, which is a pair of superconducting wires. The junction was incorporated into to a very sensitive magnetometer as well as a computer logic switch. With this critical component, a superconductor supercomputer is possible.
All of the original superconductors require extremely low temperatures that are impractical in most environments. Beginning in 1986, a new category of "high temperature" superconductors began to emerge. A particular yttrium-based perovskite mineral retains its superconductivity up to a temperature of -181°C. This is significant, because it is 15° above the temperature of liquid nitrogen, which can be manufactured easily. The operating cost is therefore much less.
Superconducting logic switches make very fast digital computers. With no electrical resistance, the computer's electronic components do not generate heat as they operate. The electronics can therefore be operated faster and can be packed closer together.
Extremely large superconducting magnets are possible. These are incorporated into MRI (magnetic resonance imaging) machines, fusion reaction chambers, particle accelerators, and magnetic levitation trains.
Zero electrical resistance permits some very accurate sensors. These include radio receivers and particle detectors.
An elusive possibility is room-temperature superconductivity. This would be any material that exhibits the phenomenon at conventional temperatures of 0°C and above. Such materials would require very little in the way of supporting machinery, but could revolutionize our entire electrical industry. Power lines could operate at much lower voltages, power tools and appliances would generate far less waste heat, and almost every known device would operate more efficiently.