Superconductor Suspension

A popular physics demonstration since the late 1980s involves floating a small piece of high-temperature superconductor, such as yttrium barium cup rate (YBa₃Cu₃O₇), over a strong permanent magnet. The levitation is easy to see, and the suspended superconductor rectangular solid spins rapidly about its long axis. The demonstration is done by first cooling the superconductor in liquid nitrogen and then using tongs to place the piece in the air above the permanent magnet. The repulsive force between the magnet and the superconductor is a demonstration of the Meissner effect. Or is it?

Answer

The demonstrated superconductor suspension does not illustrate the Meissner effect. Instead, this demonstration depends on the persistent eddy current in the zero resistivity superconducting material induced by the magnet. The eddy current direction is determined by Lenz’s law to produce a magnetic field that ends up causing a repulsion between the superconductor and the permanent magnet.

To show the Meissner effect, the sequence of events must be different. Place the superconductor on the magnet at room temperature first, and then cool the superconductor below its critical temperature T_c. Then the magnetic flux will be “expelled” by the Meissner effect and the superconductor will become suspended above the magnet.