Re: simple magnets question

[William Beaty <billb@ESKIMO.COM>]

On Sun, 27 Jun 1999, brian whatcott wrote:

> I assume that William has in mind a version of this experiment,
> so the magnet in question is a horseshoe which straddles the disc.

Actually I'm still talking about the axially-magnetized "spinning disk"
magnet from before.  This as opposed to your speedometer disk-magnets,
which must have multiple poles on the face they present to the metal disk.
If the speedometer's spinning disk-magnet has just one pole covering its
face, then it won't induce continuous currents in the metal, and it can't
drag the metal along.

We can see this when we levitate a small cylindrical rare-earth magnet
over a hunk of hi-temp superconductor.  The magnet is easily spun on axis.
(Probably most of us here have seen that the magnet will even spin by
itself because of a heat-engine effect.)  But if we try to flip it end
over end, it's like pushing it through taffy.  The type-II superconductor
presents resistance to changing fields, but doesn't present any drag if
the fields aren't changing, so it doesn't stop a disk magnet from
spinning, as long as that magnet is magnetized axially.


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