This site from the Georgia State University provides a good introduction to and illustration of the Meissner Effect, the property of a superconducting material whereby it actively excludes a magnetic field from within its interior.

Wikipedia.com offers a wonderful introduction and explanation of the Meissner effect, explaining how this effect qualifies superconduction as a true thermodynamic state of a material.

Walther Meissner discovered while working with Robert Ochsenfeld that superconductors expel relatively weak magnetic fields from their interior and are strongly diamagnetic. This phenomenon, commonly known as the Meissner effect or the...