Re: jumping ring demo

[William Beaty <billb@ESKIMO.COM>]

On Mon, 12 Apr 1999, Herbert H Gottlieb wrote:

> When energized by an alternating current, the coils in the electromagnet
> set up a changing magnetic field that induces a changing current the
> aluminum ring. According to Lenz' Law, the direction of this
> instantaneous current in the alumininum ring is such that it creates a
> magnetic field that opposes the magnetic field of the coils.

One source of confusion: a transformer is not just a pair of inductors.
When the aluminum ring is pushed down onto the bottom of the iron core,
the coupling between the ring and the coil is high, and the system should
act like a transformer with a shorted secondary: voltage and current
should be almost at 0-degrees phase in both the ring and the coil.  The
system acts like a low-ohms resistor as long as the frequency doesn't go
too low.  On the other hand, when the ring is removed from the coil
entirely, the coil should act like an inductor, where current lags voltage
by 90-degrees.

Jumping rings are an example of the non-intuitive AC stuff that Thomas
Edison disliked so much (while Tesla/Westinghouse went wild with it.)

A visual/intuitive version of the phenomena: flux lines cannot instantly
penetrate conductive objects, instead they must "ooze into" the metal.  If
we suddenly subject a metal plate to a magnetic field, the field stays
outside of the metal, and large circular currents appear in the metal
surface.  After a short time, the resistance of the metal causes the
current to decay, and the flux pushes its way into the metal.  If the
metal is superconductive, the current never decays, and so the repulsion
lasts forever.

Boeing Corp uses the "jumping ring" effect as both a sledge hammer and a
dent puller.  To make dents in metal panels, simply discharge a kilojoule
capacitor bank through a pancake-coil wound from copper pipe (water
cooled, of course.)  The expanding field gives the metal sheet a huge
impact.  To *pull* dents, simply run a hefty direct current through the
coil for a few hundred milliseconds, and then discharge the capacitor
through the same coil with polarity of current opposite that of the DC.
The DC pushes some flux into the metal sheet, and then the large capacitor
current causes an attraction force.  Or so I've been told.  If this is
true, then perhaps the shape of the discharge waveform must be tailored so
that the attraction force dominates.

I saw an interesting variation on this at Palais Decoubert in Paris (big
National science museum.)  They had an AC maglev system which lifted a
1-meter aluminum pizza tray about 70cm into the air.  It was running at a
couple of KHz, and it put out an intensely loud audio tone (maybe from the
coils, or maybe from the aluminum tray.)  Then the demonstrator placed a
brass candlabra on the pizza tray, and hooked it to a hoop-coil layed on
the tray.  The tray still levitated (at about half the height as before),
and the several light bulbs at the tops of the "candles" lit up!

I wonder how many credit cards that demo erased over its lifetime.  People
with metal surgical implants would be wise to stay clear and avoid
inductive heating, to say nothing of those with pacemakers!


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William J. Beaty                                  SCIENCE HOBBYIST website
billb@eskimo.com                                  http://www.amasci.com
EE/programmer/sci-exhibits          science projects, tesla, weird science
Seattle, WA   206-781-3320          freenrg-L taoshum-L vortex-L webhead-L