Levitation of a magnet above a cooled superconductor, the Meissner Effect,has been well known for many years. If a superconductor is cooled below its critical temperature while in a magnetic field, the magnetic field surrounds but does not penetrate the superconductor. The magnet induces current in the superconductor which creates a counter-magnetic force that causes the two materials to repel. This can be seen as the magnet is levitated above the superconductor. Keep in mind that this will occur if the strength of the applied magnetic field does not exceed the value of the critical magnetic field (H) of the superconductor. If the magnetic field becomes too strong, it can penetrate the interior of the material and lose its superconductivity. In addition the force of repulsion must exceed the weight of the magnet.
The magnet should remain suspended until the superconducting pellet warms to above its critical temperature, at which time it will no longer be levitated. It may either settle to the pellets surface or "jump" away from the pellet.
This demonstration can also be done by placing the magnet on top of the superconducting pellet before it is cooled in the liquid nitrogen. The magnet will levitate when the temperature of the superconductor falls below the critical temperature (T).
Another interesting phenomenon can be observed, while the magnet is suspended above the superconducting pellet, by gently rotating the magnet. The rotating magnet acts like a frictionless bearing as it is suspended in the air.
Demonstrating the Meissner Effect to a large class sometimes may prove to be difficult. One way to help students better see the effect is by placing the superconductor into a plastic petri dish which is resting on a styrofoam cup filled with liquid nitrogen. See Figure (18). Other methods to address larger audiences can be accomplished by the use of video projection or the use of an overhead projector. By placing an overhead projector on its side such that it still projects onto a screen, an image of the levitated magnet can be seen. Place the superconducting disk with the levitated magnet in front of the glass plate so that its silhouette can be seen. The magnet should be as close to the glass plate as possible for best results. Another method for projecting an image of the levitated magnet can be accomplished by setting up your demonstration as shown in Figure (19). Position mirrors so that the total distance from the upper focussing lense and reflector to the levitating magnet is the same as to the glass plate normally used for transparency support.
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