Superconductivity is a phenomenon observed in several metals and ceramic materials. When these materials are cooled to temperatures ranging from near absolute zero (-459 degrees Fahrenheit, 0 degrees Kelvin, -273 degrees Celsius) to liquid nitrogen temperatures (-321 F, 77 K, -196 C), they have no electrical resistance. The temperature at which electrical resistance is zero is called the critical temperature (Tc) and varies with the individual material. For practical purposes, critical temperatures are achieved by cooling materials with either liquid helium or liquid nitrogen. The following table shows the critical temperatures of various superconductors:
Because these materials have no electrical resistance, meaning electrons can travel through them freely, they can carry large amounts of electrical current for long periods of time without losing energy as heat. Superconducting loops of wire have been shown to carry electrical currents for several years with no measurable loss. This property has implications for electrical power transmission, if transmission lines can be made of superconducting ceramics, and for electrical-storage devices.
The classic demonstration of the Meissner Effect. A superconductive disk on the bottom, cooled by liquid nitrogen, causes the magnet above to levitate. The floating magnet induces a current, and therefore a magnetic field, in the superconductor, and the two magnetic fields repel to levitate the magnet.
Another property of a superconductor is that once the transition from the normal state to the superconducting state occurs, external magnetic fields can't penetrate it. This effect is called the Meissner effect and has implications for making high speed, magnetically-levitated trains (see How Maglev Trains Will Work for details). It also has...