What it shows:
A superconducting material in the presence of a magnetic field excludes that field from its interior. This is shown by levitating a magnet above a high temperature superconductor.
How it works:
We have a 25mm disc of ceramic yttrium barium copper oxide YBa2Cu3O7 that becomes superconducting above liquid nitrogen temperatures (Tc = 90K). Using a cubic neodymium magnet 4mm of side, two effects can be shown. Firstly, the Meissner effect itself, by placing the magnet on the disc before cooling. As the disc cools past its transition temperature, it expels magnetic flux and this levitates the magnet. Alternatively, cool the disc so that it becomes superconducting, then introduce the magnet. The change of flux sets up a circuital persistent current in the superconductor, whose own field repels the magnet; the result again is the magnet bobbing a few mm above the surface of the disc.
Setting it up:
The superconducting disc sits in the bottom 5mm segment of a Styrofoam cup, which itself rests on a whole inverted cup; thus is well insulated as well as providing a white backdrop for the black ceramic. Of the two processes described in How it Works, the second is usually employed so the magnet is already floating by the time the lecture begins. This aids the demonstrator in getting the best camera angles, but it does mean the audience isn't actually witnessing the Meissner effect! The disc needs to be just covered by liquid nitrogen and kept covered by topping up now and again; the magnet is placed using tweezers. The camera used is a small CCD with 10mm wide angle lens.
This is a commercially available ceramic superconductor. 1 Sergent-Welch and Cenco also sell kits. The magnet is colored black and white so the lecturer can spin it, showing clearly that it's free.
Meissner, R.W., Ochsenfeld, R., Naturwissenschaften 21, p.787 (1933)
1 Colorado Superconductor Inc., P.O. Box 8223, Fort Collins, Colorado 80526