Re: Error in Wikipedia article: Faraday's law of induction

On Jul 12, 9:18=A0pm, phil-news-nos...@[EMAIL PROTECTED]
 wrote:

> That's an interesting one I had not seen. =A0I looked at one patent
Tesla
> got that involved two disks edge to edge connected by some kind of belt
> to make it easier to extract power. =A0Back to Google.

Yes, the dual Faraday generator used flanged disks with a copper mesh
belt around them.  This places (if you choose magnetic polarity
correctly) the two generators in series which doubles the voltage (I
think Tesla had the idea of a series of these to get voltages to
useful ranges) and also lets the brushes be on the disk shafts rather
than sliding on the disks (which has lots of friction and losses at
the low voltages and high currents).

The spiral one was interesting too in that he was trying build a "self-
excited" Faraday generator. The idea was that the high currents in the
spiral rotor automatically generate a stator field without a field
supply or permanent magnets.

> Are you saying this in terms of an argument one may face and have to
pond=
er
> or are you saying that the OPFHD really produces no EMF at all?

The OPFHG actually produces voltage (which seems rather amazing at
first) But if you use Lorentz arguments you have to say that since
there is no relative motion between the magnet and disk there can be
no Lorentz voltage.  Thus EITHER the field rotates with the magnet and
ALL the output voltage comes from the non-disk part of the circuit OR
the field does NOT rotate with magnet in which case the voltage comes
from the disk [no relative motion then between magnetic field and
external wiring].

> There is the counter argument that if the magnets rotate, and the disk
> does not (wires are attached to measure), then this should induce some
> voltage in the attached wires as well. =A0Yet this configuration nets no
> EMF.

The actual counter argument actually involves the case with the disk
stationary and the brushes and meter are moving around the disk. And
that DOES produce a voltage.


> If the OPFHD really produces no EMF at all, then there was no paradox?

But is does produce voltage. In fact the SAME voltage as if the
magnets didn't move.

> I plan to buy some strong neodymium magnets to try things with when I do
=
pick
> an experiment to try.

Me too! But right now I"m using Speaker magnets. One experiment is to
encase the whole thing in an iron magnetic circuit (as Tesla did)
Iron elements complicate the picture but I'm wondering what the effect
of such flux concentration would be on output voltage and current.
Especially if you spin the disk, magnets AND iron circuit!  The thing
I'm building needs a bit more machining before I try it.

> I don't know what you mean by "measurement cannot be made with a loop".
=
=A0I am
> not suggesting some kind of loop coupling to sense the EMF in the
disk/wh=
eel.

It means that if you are measuring output using a meter and a
"circuit" there is a loop of wire there and you MUST consider the
ENTIRE loop for Lorentz forces.  Consider the following: A spinning
magnet. a "Loop" consisting of a wire down the axis of the disk shaft.
A wire vertically up the face of the magnet. A wire away from the
magnet following a "line of flux" and finally a vertical wire back
down to the shaft where it is twisted with the other wire and goes to
the meter.  The standard thought is that if you make the flux line
wires long enough, the downward wire will be in such a weak field that
you can ignore any induction  there. That is totally wrong!

Dig. The wire down the shaft follows a fluxline and has no induction.
The upper wire following a flux line has no induction. The twisted
wire pair has no induced voltages. The wire up the magnet face will
have the Lorentz force you expect due to the relative motion of the
magnetic field and the wire.  So far the experiment has everyone fat
and happy. But wait. If you look closely, you'll find that the SAME
flux lines that cut the wire on the magnet face ALSO cut the DOWNWARD
connecting wire and do it in the OPPOSITE sense. You can show the
induced voltages are equal and opposite!  Hence you can experiment
until you are blue in the face and you get NOTHING! And that also
includes no conclusions!  So long as there is a loop in the field you
simply can't get around this cancellation! It turns out the loop shape
makes no difference. This is why people suggest using electrostatic
measurements which do NOT use a closed circuit to measure emf.

> The loop is to generate a larger EMF potential. =A0Putting the light on
t=
he
> wheel itself is a means to directly measure without using brushes on the
> disk.

Tried it. But if you think about it, you see that wires going to the
light are the part of the light-loop! No light is seen.

> I'm also considering the drum configuration. =A0The conductors would run
=
along
> the drum parallel to the axis. =A0The field would be radial to/from the
a=
xis.
> And of course the motion would be circular around the drum.

Yes the drum configuration is good,  See the old EM books by E.G.
Cullwick who derives a lot on the drum and notes the "meter and
brushes" relative motion thing.

Good luck!

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