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Re: Gore's Ball




Although I have seen this effect addressed as "problems" in physics
textbooks,
it would make an interesting demo. It is certainly worth publishing in
THE
PHYSICS TEACHER.

Herb Gottlieb


William Beaty wrote:

A variation on a "rail gun" uses the magnetic fields produced by an
electric current to cause an object to
move. Brad Shue does this with a Pasco magnet, 2 brass rods and a
slider
that slides down the rods when current from a battery is ap[plied.

The force from a current in a loop of wire makes the loop expand,
and if a roller is placed on a pair of rods connected to a battery, the
roller will move away from the battery such that the area of the loop is
increased. If a kilojoule/kilovolt capacitor bank is the power
supply, hide behind thick walls before throwing the switch!

The effect is different (and stronger) if the roller/cylinder is
itself a magnet, with alike poles on the end faces and an opposite pole
on the
cylindrical surface. Direction of rotation then is dependant on
polarity. This constitutes the opposite of Faraday's homopolar disk
generator: a
homopolar motor! Cylindrical homopolar devices are topologically
identical to two stacked Faraday Disks on a common shaft, with brushes
only on the edges of the disks, and b-fields along the disk axes and
one magnetic pole between the disks. I encountered these things in
Charles Yost's ELECTRIC SPACECRAFT JOURNAL, which publishes weird and
interesting discoveries made by "fringe" researchers.

I built one tri-polar homopolar roller by gluing two .75"dia x .375"
Neodymium disk magnets together with alike-poles facing, wrapped with
tape to increase the diameter, then jammed within a short copper pipe.
(If
I ever dropped the thing, the magnets would probably launch out like
bullets!) If I recall, it took 10 or 20 amperes to make it roll along
the rails. The force was constant, so when I tilted the rails at an
incline, the roller would sit still, or would drift along if bumped, as
if
local gravity was at an angle. Odd to watch.

The device in the ESJ was a pair of parallel steel disks an inch
apart, disk planes horizontal, with the edges acting as the "rails" and
the
rollers adhering to the steel. Several tripolar homopolar rollers
were placed to link the disks together. The rollers repel each other
and
adopt uniform spacing. When many amps is run from disk to disk, the
rollers
collectively drift along.

Why do all this? BECAUSE! ;)

One thing I always meant to try: build two tri-polar cylindrical
rollers, place them on parallel conductive rods at enough distance so
their
fields do not interact, then push one along and see if the other
responds.
One will act as a homopolar generator and create large amperage at
fractional voltage, the other is a homopolar motor. How efficient are
they, how
close is the in/out coupling? The rods would probably have to be
wetted with mercury in order to eliminate the large resistance at the
contact
points. If big ones were used as the wheels in each car in a train,
then the cars would need no physical coupling, and if one was dragged
along, all the others would move as well.

.....................uuuu / oo \
uuuu........,.............................
William Beaty voice:206-781-3320 bbs:206-789-0775
cserv:71241,3623
EE/Programmer/Science exhibit designer
http://www.eskimo.com/~billb/
Seattle, WA 98117 billb@eskimo.com SCIENCE HOBBYIST web
page