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The Meissner-Ochsenfeld effect

Consider a perfect conductor with zero magnetic flux inside it. If a permanent magnet were brought near the perfect conductor, the magnetic flux inside the conductor would remain zero, due to the induced currents. The magnetic field from the induced currents would oppose the magnetic field of the permanent magnet. If this permanent magnet had a magnetic field strong enough to support its own weight, it could be levitated above the perfect conductor. In Berlin in 1933, an experiment was done by Walther Mei▀ner and Robert Ochsenfeld to test whether superconductors behaved the way Maxwell's laws predicted a perfect conductor to behave. They found that a superconductor will expel any magnetic flux whether it becomes superconducting before or after the magnetic field is brought near it. A perfect conductor would resist a change in flux, while a superconductor expels any flux at all. The explanation for this is that a superconductor is not a theoretical perfect conductor as described by Maxwell's laws. A superconductor is a perfect diamagnet, a material which resists internal flux.
next up previous contents
Next: Josephson Junctions Up: Introduction Previous: The discreteness of the   Contents
Barry Fitzgerald 2001-05-17