Hello everybody. This is an experiment in
electromagnetics and we're studying a phenomenon called Faraday's paradox, a
phenomenon discovered in 1851 by Michael Faraday the father of electricity
So we've got
two objectives - one is to demonstrate a Faraday paradox which is a simple physics demo,
and the other is to investigate a paper made by A.G. Kelly in
1998. A.G. Kelly claimed that there were some things that Faraday missed when he did his original
experiments. Now the A.G. Kelly paper is here and
I've got a copy of this on the website which is
given in the bottom of this video.
So you can go and get a copy of the paper and also you can see a
biography of A.G.Kelly if you so wish
So anyway moving on to the experiment:-
What we have here is a repurposed electric drill acting as a motor which will rotate a disc which is this disc here and the
disc is rotating in front of a bar magnet
this bar magnet is a ceramic magnet out of a loudspeaker and I've put a couple of pole pieces on it.
Mild steel pole pieces one here and one here and this is an exact replica of the magnet that Kelly used for his
experiment and so
when the disc rotates in the magnetic field it should give us an emf
generated across the disc and that will be recorded on the meter here so let's fire it up and see what we get.
I start the motor
and you see it doing about 580 RPM
And you can see that the generated emf is about 3.9 millivolts or so and that's a straightforward
Faraday disc and
nobody disputes that this is how it works.
Where we get to the interesting part is if we rotate the magnet at the same time as
we do rotate the disc
Now you would expect this would not generate an emf because now, rather than a disc cutting the lines of force of
the field, both the disc and the magnet are rotating together so there's no
relative motion of the conductor through the field. So there should be no emf generated. So I'm going
to shut this down and lock the magnet to the shaft so as to co-rotate the magnet with the disc and we'll see what happens.
Okay i have now locked the disc and the magnet to the same shaft,
so you can see as i move it back and forth that the magnet and the disc co-rotate.
So now you would expect that when I start the motor we would not observe an emf because the
magnet and the rotor are going together so there's no
relative motion, and you can see the brush here is where the emf is picked up.
One contact is on the brush the other contact is on the metal bearing in which the shaft resides there.
So let's start this up and see if we get an emf or not.
Surprise, we DO get an emf! We are doing
625 RPM and we've got about three point nine
millivolts of emf so that's about the same
emf per
unit rotation that we had before.
So this is the Faraday paradox. Faraday first observed this and
this is duplicating his experiment of 1851.
Now the question is how can this be? How can it be that you get an emf even though the magnet's rotating as well?
Faraday came up with the idea that the magnetic field does not itself rotate but rather is fixed in space.
He himself wasn't very satisfied with this explanation
but that's the explanation he gave. We have some more modern explanations of the Faraday paradox
which are based on the way in which Maxwell's equations are
interpreted and so if you put the correct interpretation of Maxwell's equations it is possible to predict that this will happen.
But what you must remember is that this was one of the original
experiments on which Maxwell's equations were based and so it's not really surprising that it obeys Maxwell's equations.
so anyway,
that's what Faraday found.
Now let's investigate what Kelly said and Kelly said that if you route the magnet if you route the
wire through the magnetic field that's rotating then that will change the emf that we measure so let's see if that's the case.
I'll start it up.
There we have about 630 rpm
4.05 or 4.1 millivolts, let's see what happens when we re route the wire.
Now this is the wire that's connecting to the brush we can see if I lead it around the magnet like this
There's a transient because I'm moving the magnet through the field but once it settles down it settles down to the same voltage it was
at before
so we can come straight out this way, and then back,
same emf or we can come straight out,
same emf. Now it was Kelly's contention that the emf changed dramatically when you do this
but I am unable to replicate Kelly's results.
It's a shame I'm unable to replicate Kelly's results because were they true it would have some very profound
implications. We could have a new energy source, we could have a new space drive, we could have - more mundane - we could have a
brushless homopolar motor which would be of huge commercial value and
for some reason it doesn't work when I do it!
Kelly reported that
altering the routing of the wire made a big difference with a co-rotating magnet but did not make a difference
when the magnet was stationary and that, you would expect
because if you put a wire in a static magnetic field it won't have any emf generated in the wire
but if you put the wire in a rotating magnetic field
then you will generate an emf in the wire and that will change the emf measured on the meter.
But it's not the case in my rig at least. I find the exact same results that Faraday himself found.
Faraday did not report that the emf changed with rerouting of the wire and I don't report that either.
So if anybody is interested I've got A.G. Kelly's original paper on my website.
Also I have a biography of the man - he was a very respected well-known engineer and
the website is at emmanuelavionics .org/faraday.htm
and there should be a link to that in the comments here to this video.
So if anybody wants to go and look at that, I've also done a proper write-up
of the experiment. So all the paperwork is available and the dimensions and things and
so
I'd very much like to hear from anybody who can replicate Kelly's results. I could not but I would love it if somebody could.
So anyway that wraps up the demonstration for today thank you for watching.
 
 
