Walther Meissner (Germany, 1882-1974) was one of the main physicists who studied low temperatures in Germany. He installed the first German helium liquefier in 1925, which enabled him to give his name to the effect he discovered in 1933, along with his colleague Robert Ochsenfeld.
They measured the magnetic field produced by a sample of tin and discovered that when it becomes superconducting, tin expels the magnetic field from its volume. In other words, the magnetic field in the middle of the superconductor is equal to zero, as if the superconductor became a Faraday cage for magnetic fields. It is this effect that enables magnet levitation.
