Wikimedia
Wikipedia: The Meissner Effect
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.
National High Magnetic Field Laboratory
Magnet Academy: Walther Meissner
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...