Superconductivity is a set of physical properties observed in certain materials where electrical resistance vanishes and magnetic fields are expelled from the material. Any material exhibiting these properties is a superconductor. Unlike an ordinary metallic conductor, whose resistance decreases gradually as its temperature is lowered even down to near absolute zero, a superconductor has a characteristic critical temperature below which the resistance drops abruptly to zero. This allows electric current to flow indefinitely without energy loss.
Superconductivity was discovered on April 8, 1911, by Heike Kamerlingh Onnes, who was studying the resistance of solid mercury at cryogenic temperatures using the recently produced liquid helium. For decades, it was thought to only occur at extremely low temperatures (near absolute zero). In 1986, 'high-temperature' superconductors (working above the boiling point of liquid nitrogen, -196°C) were discovered, which was a major breakthrough, though the mechanism is still not fully understood.
Superconductivity has profound applications in modern technology. It enables the creation of powerful electromagnets used in MRI machines, particle accelerators (like the Large Hadron Collider), and fusion reactors. It is also key to the development of quantum computers (using superconducting qubits) and potential future technologies like lossless power transmission grids and maglev trains. The search for a room-temperature superconductor remains one of the 'holy grails' of condensed matter physics.