Certain materials, when cooled below their transition temperatures, become superconducting - that is, electrical currents travel in them with zero resistance. There is no resistive heating, and if the superconductor forms a closed circuit, the current will continue to flow forever, without any voltage drop or decrease in magnitude. In this mode, superconductor circuits can serve as powerful, lightweight permanent magnets.

A detailed description of the physics of superconductivity is complex, and beyond the scope of this summary. Basically, at sufficiently low temperatures, the conducting electrons drop down to an energy level below their normal state. In this new state, the electrons can travel through the superconductor without colliding with, and losing energy to, its atomic matrix. Because they lose no energy, they can travel forever through the conductor, needing no voltage input.

Superconductivity was discovered in 1911 by Kamerlingh Ohnes, the first person to liquefy helium. Since then, there has been a continued rise in superconductor transition temperatures. High transition temperatures are desirable, because the amount of electric power input to the refrigerator that keeps the superconductor at low temperature decreases as transition temperature increases. For example, at 4.2 degrees Kelvin, the normal boiling point of liquid helium, to keep the superconductor cold, approximately 500 watts of electrical power is consumed by the 4.2 K refrigerator to remove one watt of thermal heat that leaks in through the surrounding insulation. (4.2 degrees Kelvin is equivalent to minus 459 degrees Fahrenheit - a very cold place indeed.)