The Meissner effect is the phenomenon in which magnetic fields are expelled from a superconductor when it is in a superconducting state. It was discovered in 1933 by German physicists Walther Meissner and Robert Ochsenfeld. Their process to discovery included measuring the magnetic field distribution of superconducting tin and lead samples. In the presence of an applied magnetic field the samples were cooled below their critical temperature (hydrogen2oxygen.net). The samples expelled nearly all magnetic fields below their critical temperature. The experiment demonstrated that superconductors are not just perfect conductors, and provided a defining characteristic of superconductors.
BCS theory (Bardeen, Cooper, and Schrieffer), describes the phenomenon of superconductivity as a formation of pairs of electrons. These pairs are known as Cooper Pairs, the "C" in BCS theory. A superconductor expels mostly all magnetic flux, which is the amount of magnetic activity through a material. The flux is not completely canceled at the surface due to electric currents near the surface. The currents cancel the applied magnetic field within the superconductor.
Superconductors exhibit perfect diamagnetism known as super diamagnetism, so the total magnetic field is very close to zero. It isn't typical diamagnetism which is directly related to orbital spin. Super diamagnetism occurs because the superconductor's currents flow opposite of the applied magnetic field.
