Re: simple magnets question

[brian whatcott <inet@INTELLISYS.NET>]

At 19:45 6/25/99 -0700, Bill wrote:
> If a spinning Faraday-disk magnet is held near a metal plate, there should
> be a zero current (but perhaps there will be a brief, tiny radial current
> when the plate is first brought close to the magnet.)
...
> William J. Beaty

Faraday says (para 3159 Series 28 oct 1851 in part)
"...I had an apparatus constructed fig 19 consisting of a wooden axis
one extremity of which was terminated by a copper screw intended to
receive and carry one or more discs of metal that might be screwed onto it.
This end projected so far beyond the support that such discs could be
partly introduced between the poles of a horseshoe magnet, so as when
revolving to move across the lines of force at their most intense place of
action...
One of the galvanometer wires was pointed [and held against]the end of the
axial screw. And the other was applied by the hand  or so fixed as to bear..
against the rim of the disc"
(Fig 19 shows a handle affixed to the other end of the axle.)

I assume that William has in mind a version of this experiment,
so the magnet in question is a horseshoe which straddles the disc.
 When he confidently asserts there will be a zero current, he evidently
refers to an external current through the electrodes described
here by Faraday.

In contrast but not in contradiction, I confidently assert that
such a disk held fixed while a horseshoe rotates about it will
certainly get warm as a result of the eddy currents in the disc.
This is not a difficult arrangement to contrive.

   (In reviewing the many pneumatic tools on offer, I see a
"pneumatic die grinder" which is an air driven collet contrived to
spin small stones at about 340 revolutions per second.
This might work well!)

brian whatcott <inet@intellisys.net>
Altus OK