THE WEIRD BUBBLE
FARADAY HOMOPOLAR GENERATOR
It is unlikely that you
will find the description of the Faraday homopolar generator in any
university textbook on electromagnetism. The closest you will come
across is the plain homopolar generator. The fundamental
difference is that in Faraday's device the magnet generating the
magnetic field moves or rotates together with the conductor where the
voltage is supposed to be generated. Its operation was first reported by
Faraday in 1831 and fig. 1 shows a practical implementation of the
discovery. A metal disk, 3 or 4 times the diameter of a ring magnet, is
glued to it and the combination is rotated while the voltage is measured
between two sliding contacts, one placed at the center and another is
allowed to slide from the center up the edge of the copper disk. The
maximum reading takes place when contact A is at, or just beyond, the
radius of the magnet. Actually, you will read almost the same voltage
regardless whether the contacts are between the center and the radius of
the magnet or between the radius of the magnet and the outer
edge of the disk. Obviously the first solution is a
more practical implementation. As a generator it seems to have only
drawbacks: the output voltage is generally rather low, just more than 2
mV with my set up using a 4VDC electric motor rotating between 20 and 40
rev/sec and a ring magnet of 36mm diameter. Other experimenters have
reached hundreds of mV or more with powerful magnets and motors. It is a
DC machine and the presence of sliding contacts introduces a voltage
drop which could make the generator totally useless given the high
current expected. Yet, there are some rather puzzling aspects concerning
this generator: as the magnet is moving together with the copper disk,
you would expect that no voltage is generated. In fact, the voltage
is definitely there and it is the same as you would have with a
stationary magnet and a rotating disk. This latter case could be
explained with the law of induction and we could figure out that when
the disk and the magnet are rotated together we have a voltage
generation in the wire connected to the brush; but this happens
regardless of its orientation, its size, reduced to a needle and even
with the wire magnetically shielded, except for the contact point.
However the real issue surfaces when we consider its operation as an
electric motor: we should expect a rotation of the disk/magnet
combination after a certain voltage is applied across the sliding
contacts. From a practical point of view it will be quite difficult to
send, say, hundreds of Amps through the sliding contacts but is the
theoretical implication that remains without a proper answer: against
what is the magnet/disk moving? The only solid material available are
the sliding contacts but the direction of the current is such that that
disk appear to exert no force on them for a
certain orientation or if they are shielded. Is the disk/magnet moving
without a counterforce? Conversely, if we use the device as a generator
and we apply a load, we are unable to see where the back-torque is
exercised and the generator seems to generate electricity without a
corresponding input loading, except for friction losses. Because of the
implications one would expect a rush of experiments but there is little
activity in this area and only a handful of people are or were involved
with these machines.
From an experimental point of view there are other configurations that should be explored, with the secret hope to find a clue towards a better understanding of the underlying mechanism.
If we take a common cylindrical metal magnet we can dispense
with the copper disk. There is a voltage between either of the magnetic
poles and the center point of the magnet. (Fig. 2) The exact
position and shape of sliding contact B at the magnetic
pole is not critical although you get a higher voltage when the contact is
placed off axis or right on the edge. The voltage will decrease if contact
A is moved away from its center position
between the poles until you will have no voltage when the contact reaches
either end of the magnet. The maximum reading was 2.4mV using the same
electric motor as in the previous experiment with a magnet 25mm long and
6mm diameter. The same considerations on back-torque, operation as an
electric motor, etc. could be applied to this configuration yielding the
same set of unanswered questions. The major difficulty in generating a
usable amount of energy is to have a very low resistance for the sliding
contacts: their resistance must be close to zero or else the voltage drop across the
contacts could easily be equal to the generated voltage leaving nothing
for an external load. This can be achieved only with the use of sliding
contacts lubricated with mercury. An interesting consequence of this experiment is that the Earth,
a rotating magnet, should have similar properties: an electric field of
equal polarity at the magnetic poles, the pink area, and the opposite
polarity around the equator, the greenish belt. The sliding contacts could
very well be the stream of charged particles coming from the sun thus
creating a huge homopolar generator. A more detailed drawing would show
that also the polar region is doughnut shaped with a small deep at the
very center of it.
The low voltage, typical of this machine, can be increase by simply placing another magnet, with the south pole facing the copper disk of fig. 1, in such a way that the disk is in the middle. This solution will double the available voltage. An additional experiment was conceived with the purpose of providing a different mechanical configuration (fig. 3).
Two ring magnets of the same type used in the
first experiment were assembled so that the same magnetic pole face each
other. The distance was about 20mm and a copper sleeve was placed in
such a way to slightly overlap the like poles of the two magnets. The
voltage was taken right at the edge of the sleeve as shown in the
drawing and also in this case you will measure twice the voltage. A
benefit of this solution is that both sliding contacts are mechanically
the same: this could be an advantage from a mechanical point of view as
there is no need to connect electrically to a center point. In all the
above experiments was observed no decrease or variation of the voltage
in spite of the magnetic screening, in the form of steel or ferrite
tubing, which was applied to the wires connecting the brushes. In order
to avoid eddy currents the brushes were reduced in size until they were
just two needles without any change in the output voltage.
The fact that the Faraday homopolar generator is a DC machine is an inconvenience that limits its flexibility. Fig. 4 is a suggestion of a machine generating an alternating current.
A
square magnet, with north and south poles on the sides, is rotated and the
voltage is taken with the collector brushes placed at the edge of the
disk. If one of the brushes is placed in the center you will have
half the voltage. One major problem with this generator is that there is
also an induced voltage on the collector wires. This means that if you
wish to measure the homopolar voltage you should try to get rid of the
induced voltage. This was found to be rather difficult and screening the
collector wires with ferrite tubing did help only up to a certain extend,
I was not able to totally eliminate the induced signal which was
interfering with the signal generated with the homopolar effect. There
seems to be also a 90 degree phase difference between the two signals but
further investigation proved difficult due to the low level of the signal
and the precarious mechanical set up I was using. A variation of the above
AC generator was implemented using the rotor of a bicycle dynamo, an 8
pole magnet. Its outer circumference was covered with a layer of copper.
The brushes were placed at a distance of 45 degrees, or odd multiples of
it, one from the other, this is the same distance of one magnetic pole
from the other. The disturbing signal from the induced voltage is even
more difficult to control and careful magnetic screening and positioning
of the collector wires was necessary in order to detect the homopolar
voltage.
As expected the output voltage depends on the speed of the rotor and the intensity of the magnetic field. What was not expected was the fact that the largest voltage is present when one of the brushes is placed where the magnetic field is most inhomogeneous while the other brush is located where the field is mostly homogeneous or absent. A special case is the machine of fig. 3 where both brushes are placed within an inhomogeneous field but they both refer to a zero potential which is a point in the middle between the two brushes. Actually it was found that there is an ideal distance between the magnets of fig. 3 where the voltage is the highest; it is likely that the magnetic lines are more or less deformed and hence the resulting field is more or less homogeneous.
The results of all the above experiments seem to obey the
law whereby a conductor generates a voltage across it when it moves in
non-uniform magnetic field (fig. 5b). Let us repeat the basic
Faraday experiment (fig. 5a) but this time we employ one
cylindrical magnet (two are shown in order to have a mechanically
balanced system) providing a magnetic field that does not extend on the
whole disk. The sliding contact is next to the magnet. Rotation of the
disk will give the same voltage as in fig. 1 but rotation of the disk and
the magnets does not give the same voltage but a pulsing
unidirectional voltage, once you manage to remove the induced signal. It
is now the wire connected to the sliding contact that is immersed in a
non-uniform magnetic field that generates the homopolar voltage. The
original Faraday experiment (fig. 1) with the disk and the magnet
moving together is a very special case because we could think that
it is the sliding contact that is actually moving in an inhomogeneous
magnetic field and becomes the source of a continuous voltage because
the field is always present. In fig. 5b we could move together the
conductor and the magnet originating the field but we would still have a
DC voltage because this is the same as having a stationary
conductor/magnet and the sliding contacts moving in a non-uniform
magnetic field. We now know where the voltage is generated and any
magnetic shielding of the wire will still preserve the non-uniformity at
the contact point, so it will make no difference. Surely it will make a
difference when the mechanical forces acting on the various parts are
considered but we have no satisfactory answers, as yet, and further
experimentation is necessary.
References and links:
1) 1) http://www.marmet.org/louis/induction_faraday/index.html (French site)
2) http://depalma.pair.com/Tewari/Tpatent.html
3) http://depalma.pair.com/index.html
4) Di Mario, D. 2001, Faraday's Homopolar generator, Electronics World, (vol. 107-1786), Highbury Business Communications, Cheam, UK
5) Marinov, S. 1995, On the fundamental law in electromagnetism, Speculations in Science and Technology, (vol. 18-2), Chapman & Hall, London
6) Mencherini, L. 1993, Relativistic interpretation of kennard's and Müller's experiments on the unipolar induction phenomenon, Speculations in Science and Technology, (vol. 16-2), Chapman & Hall, London
7) Martin, T. (editor), 1932, Faraday's Diary, Para. 255-257, Bell.
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