Re: "Faraday's Disk" which started it all

[brian whatcott <inet@INTELLISYS.NET>]

At 13:02 7/3/99 -0400, Bob Sciamanda wrote:
> There is a measurable difference
> between relative rotation and "absolute" rotation (take this latter phrase
> to mean rotation relative to the fixed stars).  To paraphrase Mach, you can
> only produce a curved meniscus [paraboloidal] surface in a pail of water
> by rotating the
> water in  this "absolute" sense; you will not observe this meniscus if you
> leave the water still and put only yourself into a spin, while standing on
> the extended axis of the pail.  The spinning water develops a meniscus only
> when IT is rotating, whether you are rotating or not.
>
> Events occur among the constituent particles of water when the water is
> spinning which do not occur when it is not spinning.  This has to do with
> properties of the water (internal stresses and the water's response thereto)
> which come into play when it rotates - this is part of the physics which SR
> cannot be expected to divine simply from its stress free behavior in
> translation.
>
> If relativity alone determined the details of rotational behavior, it would
> be identical for ALL rotating materials (water, steel, etc); there would be
> no room for variation in the relevant properties of different materials.
> (In the same way, beginning students will often mistakenly expect
> conservation of momentum/energy alone to determine the details of a
> collision in the general case, allowing no room for a variety of results
> from a variety of interaction mechanisms.)
>
> The same must be said of a spinning magnet.  Its electromagnetic properties
> change, TO ALL OBSERVERS, when IT spins.  Internal OBJECTIVE events occur
> which do not happen in a non rotating magnet.   No purely kinematical
> transformation theory can divine this behavior unless you feed it the
> particular PHYSICS of the phenomena involved under rotation conditions.
>
> -Bob
>
>
> Bob Sciamanda

This post (and the one before it concerning the surface bound
charge artifice) counts for me as a view of the mountains above the
cloudscape hanging beneath, an excellent insight that supplies details
that were not so visible at lower altitudes.

  One acknowledged the 'absolute' role of rotation, but the
spinning water illustration was new.
 Hehehe.. to place Special Relativity and not just Lorentz
in the role of a 'kinematical transformation theory' was delightful.

And now for the merest soupcon of experimental support,
I report in favor of the null hypothesis when I spin first a 3 cm
diam. 5 mm thick ferrite disk at 300 revs/sec and find no current
or EMF with any of several search coils deployed radially from the
axis of rotation in any of three orthogonal axes referred to the
axis of rotation of the magnet -
or when tiring of searching for effects from this rather
 feeble magnet (capable of lifting 60 g or so) I spin instead an alnico
coaxial bipolar magnet with symmetrical field on any face radius
at similar speed with similar results.
(This magnet diam 3 cm thickness 3 cm capable of lifting 1.2 kg.,
the sensor a DVM of ordinary sensitivity)

Which reminds me - that a favorite way of transferring AC signals
through a rotary joint (say a scanning radar head) is to dispose
two coils with radial symmetry on either side of the joint -
the rotation becomes invisible.


brian whatcott <inet@intellisys.net>
Altus OK