The phenomenon in which certain materials lose all electrical resistance at very low temperatures is called superconductivity. Below a material-specific critical temperature, electrical resistance can drop abruptly to zero, allowing a current to persist without an applied voltage or ordinary power loss.
Superconductivity was discovered in 1911 by Dutch physicist Heike Kamerlingh Onnes while studying mercury at cryogenic temperatures. It involves more than perfect electrical conduction: superconductors also expel magnetic fields from their interiors, an effect called the Meissner effect. This magnetic behavior is a defining part of the superconducting state and can produce striking levitation demonstrations.
In conventional superconductors, electrons form paired states known as Cooper pairs. The paired electrons can move through the material without the scattering that normally produces electrical resistance and heat. Superconductors are used in applications such as MRI magnets, particle accelerators, sensitive detectors, and some magnetic-levitation systems.
Superconductivity is not the same as superfluidity, which describes frictionless fluid flow. Ferromagnetism concerns permanent or strong magnetization, while piezoelectricity is the production of electric charge under mechanical stress.