20 January 1989
Vol 243, Issue 4889

About The Cover

Three 12-millimeter disks of the high-temperature superconductor YBa2Cu3O7- levitating above a permanent magnet, which rests in a bowl with liquid nitrogen. The pinning of magnetic flux lines inside this superconductor causes an invisible friction, which stabilizes the levitation of the disks, strongly damps its oscillation and rotation, and facilitates free suspension of small disks below a magnet. See page 349. [E. H. Brandt, Max-Planck-Institut f&uumlaut;r Metallforschung, Institut f&uumlaut;r Physik, Stuttgart, Federal Republic of Germany]