The Meissner effect is the falling off of the magnetic flux within a superconductor (see superconductivity) when it is cooled to a temperature below the critical temperature in a magnetic field. It was discovered by the German physicist Walter Meissner (1882–1974) in 1933 when he observed that Earth's magnetic field was expelled from the interior of tin crystals below 3.72 K.

The video to the right shows an experiment to illustrate the Meissner effect in a superconductor. It was performed at the University of Twente in Enschede, The Netherlands, under the supervision of Professor Alexande.