Ordinary metallic conductors have electrical resistance, which dissipates electrical power as heat when a current flows through them. Although resistance reduces slightly as the temperature is lowered, even at close to absolute zero there is significant resistance. When a superconductor is cooled, an abrupt change occurs at its critical temperature whereby all resistance suddenly disappears. The superconductor can then carry an electrical current without dissipating any power. Current can flow around a loop of superconducting material indefinitely, acting as a perfect energy store.
The first superconductors to be discovered, known as conventional superconductors, had critical temperatures close to absolute zero. This meant that superconductivity could only be achieved using liquid helium, which has a boiling point of 269° C (7 K), and they were not practical for real-world applications. The more recently discovered high-temperature superconductors have significantly higher critical temperatures, which can be achieved using readily available liquid nitrogen, which has a boiling point of -196° C (77 K). This opens up the possibility of using superconductors in engineering applications.
Various theories have been proposed for how superconductivity occurs. The Bardeen–Cooper–Schrieffer (BCS) theory explains superconductivity as resulting from electrons condensing into Cooper pairs — pairs of electrons that bind together at low temperatures. However, this theory cannot explain high-temperature superconductivity and, despite a number of theories being put forward, there is still no accepted mechanism for how this occurs.
Superconductors have many applications, many stemming from the ability to create extremely powerful electro-magnets. These magnets are used in magnetic resonance imaging (MRI), mass spectrometry and particle beam steering. They are also being used for plasma confinement in fusion reactors, an application where superconductivity may prove of enormous value in the future.
Superconducting electro-magnets can also be used to build electric motors which have extremely high power-density, torque and an electrical energy efficiency better than 99.9%. However, the power to run the required cryogenic cooling means the overall efficiency is closer to 99%. Such motors have already been tested in wind turbines and other power generation applications. They are also seen as an enabling technology for the electrification of civil aircraft. Superconductors have other applications in power storage, regulation and transmission.