[mattlikespeople]

The Meissner effect effectively tells us that, in a weak applied field (at higher fields this is not necessarily the case), a superconductor expels all magnetic flux. Although the magnetic field is completely expelled from the interior of the superconductor, there is not a sharp transition at the edges of a sample, rather there is a rapid decay of field into the sample over a distance called the penetration depth. Each superconductor will have a characteristic penetration depth dependent on the material properties. When a superconductor is cooled in a weak magnetic field, as it crosses below the transition temperature, persistent currents arise on the surface and circulate so as to cancel the flux inside (c.f. a current flowing around a loop generates a perpendicular magnetic field - the superconductor does the same to generate a field which opposed the applied field. These persistent currents only flow in a depth equal to the penetration depth. Duh