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Hanging by a magnetic thread.

Hanging by a magnetic thread

The sight of a superconducting pellet floating above the surface of a magnet is a striking demonstration that superconductors repel magnets. Now two research groups report that it's possible to suspend a superconducting pellet below or to either side of a magnet (see photo). The observation of this unusual type of levitation suggests the presence of an attractive force in addition to the normal repulsive interaction between superconductors and magnets.

"It's a curious and interesting effect," says Allen M. Hermann of the University of Arkansas in Fayetteville. Hermann and his colleagues demonstrated the effect in a thallium-based superconducting compound (SN: 4/2/88, p.213). A team including scientists from NASA's Marshall Space Flight Center in Huntsville, Ala., has observed similar behavior in a superconductor made from yttrium-barium-copper oxide mixed with silver oxide. Both groups will report their findings in APPLIED PHYSICS LETTERS.

The unusual levitation effect appears to arise because the magnet's field penetrates certain types of superconductors, generating currents within the material. However, because the superconducting material contains impurities, these currents and the magnetic field they generate somehow become trapped. Hence, the superconductor retains this induced magnetic field. With a magnet nearby, the result is both an attractive and a repulsive interaction that combine to keep the superconductor a certain distance away from the magnet.

"To calculate that effect and make it a quantitative explanation is extremely difficult and probably impossible," says Hermann. "It's not so much a question of computational difficulty. It's that we don't know the configuration of how the [magnetic field] threads itself around inside the superconductor."
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Title Annotation:superconductors and magnets
Author:Peterson, Ivars
Publication:Science News
Date:Aug 6, 1988
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