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

On Mon, 5 Jul 1999, brian whatcott wrote:

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 -

Oh dear. There's a miscommunication here. I'd expect the e-field of a
spinning magnet to resemble the e-field surrounding an "electret." It
possibly would be measurable by an electrometer, but I don't see how it
could ever cause a significant current (except maybe for brief picoamps
when a conductor is first moved into the e-field.)

In a homopolar generator, the large current only appears when a
conductor-disk rotates in the magnetic field while the measuring circuit
is stationary (or when the measuring circuit sweeps around the rim of a
stationary conductor-disk.) In both situations, conductors move in a
b-field, so obviously q(VxB) plays a critical role. Does qE play a role
as well? To answer the question, we'd have to directly measure the E near
the spinning magnet, without employing any rotating conductors. Can a
typical electrometer reliably measure e-fields of approximately 1 V/M or
below?



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


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