In this demonstration, we have placed a yttrium-barium-copper-oxygen (123) superconductor inside liquid nitrogen (-196°C). As it is cooled below the superconducting transition temperature, the material becomes a superconductor and a perfect diamagnet, expelling the applied magnetic field. Because of this, when a magnet is held above the material it starts to levitate and hangs suspended in air. Gradually when the liquid nitrogen boils off and the superconductor returns to temperatures above its critical point, the magnet eventually loses its levitation and falls.
Table tennis balls that are glued together can be stacked on top of one another showing the two most important kinds of packing – hexagonal and face centred cubic packing. One may use balls painted in different colors to bring home the ideas more vividly. Click on the images for close-ups.