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

[brian whatcott <inet@INTELLISYS.NET>]

At 19:43 7/4/99 -0700, Bill wrote:
> 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?
> ....
> William J. Beaty

Bill seems to see a categorical difference between sensors of the
electrostatic field and ordinary voltmeters which are current driven
- or used to be at least.
 These days, considering a FET input DVM, one sees a direct
electric field sensing mechanism, in my view!

At any rate, the experimental evidence was less clear cut for a
radial electric field.
 Using Faraday's approach to measuring an EMF at a spinning disk,
a field of 1 V/M would amount to 15 millivolts over the 15 mm of a
ferrite magnet radius. An average DVM hits its minimum reading at 100
microvolts.
The bulk resistivity of the ferrite specimen was very high,
but the measured voltage was not entirely consistent with a motional EMF.

 There appeared to be a considerable triboelectric component - and the
abrasive quality of the surface at the considerable peripheral speed
involved was hard on the probes. I saw values hovering around
30 millivolts in one run.
But I took no care to neutralize the geomagnetic field, which itself
provides the basis for a conventional style Faraday generator.




brian whatcott <inet@intellisys.net>
Altus OK