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I saw a terrific demonstration the other day by a superconductor physicist whom we were recording for the programme next week; she had a circle of powerful magnets arranged around the outer edge of a large disc resembling a coffee table top. Above this ring of magnets she placed a small puck (2 inch by 2 inch) of a copper oxide based superconductor cooled to -200C. The puck hovered about 2cm above the table top and, when given a push, followed the path of the magnets, in a circular path. This is relatively simple to explain, because the superconductor generates a magnetic field that opposes the field applied by the magnets and this keeps it suspended.But, the next thing she did was to turn the table upside down so now the puck was hanging under the table and following the same circular path.If the magnetic effect was purely a repulsive one, the puck would have fallen to the floor. Instead it hangs there; so it seems to be both attracted and repelled from the surface at the same time.How?
My understanding is that the superconductor completely nullifies the magnetic field within itself (Meissner effect: https://en.wikipedia.org/wiki/Meissner_effect), and that there is some (fairly small) barrier to changing its response to a change in the magnetic field around it. So the superconducting object will be free to move about within a homogeneous field, but will meet some resistance in going closer to or further from the source of the field.