The falling off of the magnetic flux within a superconducting metal when it is cooled to a temperature below the critical temperature in a magnetic field. It was discovered by Walther Meissner (1882–1974) in 1933 when he observed that the earth’s magnetic field was expelled from the interior of tin crystals below 3.72 K, indicating that as superconductivity appeared the material became perfectly diamagnetic. See magnetism.