The levitating aluminum plate is a different phenomena.  You put the
plate over an AC-powered electromagnet and the varying magnetic field
induces eddy currents in the plate.  The eddy currents flowing in the
aluminum create another magnet with the same polarity which causes the
aluminum plate to be repelled by the electromagnet.  Of course it only
works with an AC field.

Doc Edgerton, who you might know as the inventor of the strobe light,
made an underwater seismic energy source called a "boomer" which had a
metal plate snugged up against a coil.  He would charge up a capacitor
(to several joules) and then dump the charge into the coil. The large
current in the coil induced eddy currents in the metal plate which made
it bend away from the coil, push against the water, and make a pretty
good pop in the ocean.  The boomer was a relatively high-frequency
seismic source and is still used for shallow, high-resolution seismic
reflection surveys.

He made another seismic energy source which ran on the same power supply
called a sparker.  A few thousand volts from a capacitor is discharged
into an electrode trailing behind the boat.  The discharge would
vaporize the salt water creating a small explosion and of course seismic
energy.  The sparker was lower frequency than the boomer and is used for
deeper (hundreds of meters) surveys.

Doug Crice

Warren Offutt wrote:
> 
> Charles Patton's post rang a bell:   At the New York World's Fair,
> 1939-1940, I recall seeing discs of aluminum (several inches in diameter)
> levitated several inches above an AC powered magnet.
> 
> It seems to me I recall seeing this described in popular science magazines
> of the 1950s as well.
> 
> A long time ago, and memory is fallible, but I think it is correct.
> 
> warren
> 
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-- 
Doug Crice          web site http://www.georadar.com
GeoRadar Inc.             e-mail dcrice@............
19623 Via Escuela Drive           phone 408-867-3792
Saratoga, CA 95070 USA              fax 408-867-4900

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