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Suicidal glass plates


Hello all!

The other day I saw a roll of 3 Mil plastic sheeting
(10 feet x 25 feet) for sale in K-mart for about $5.00.
For the heck of it, I bought one and made a plastic/
aluminum foil sandwich capacitor out of it, using 4
sheets of the plastic between each of the sheets of
foil, resulting in a 0.13 uF capacitor after about 1.5
hours of work.

To make the cap easier to carry around, I sandwiched
it between two 1/4 inch thick sheets of glass - one
on the top and one on the bottom.  Note that the
capacitor was insulated from the glass by placing several
additional sheets of plastic between the capacitor
and the glass sheets, resulting in the following
unit:

************************  1/4 inch glass
------------------------  insulating plastic
  XXXXXXXXXXXXXXXXXXXX
  XXXXXXXXXXXXXXXXXXXX    The capacitor
  XXXXXXXXXXXXXXXXXXXX
------------------------  insulating plastic
************************  1/4 inch glass

I then sat the whole unit on the carpet in my appartment
and slowly charged the capacitor up (with DC voltage)
by using a 15 KV neon sign transformer and a full-wave
rectifier, with a variac to control the output voltage
of the transformer.  When I got up to about 12 KV (DC),
the bottom glass plate broke!  Remember that the glass
plate is not even part of the capacitor - the capacitor is
simply sitting on the plate!  I then continued increasing
the voltage, by slowly turning the knob on the variac.
Around 17 KV, the top glass plate broke!  (Note that
there is no weight piled on top of the capacitor, either.)
Furthermore, I saw no corona anywhere (like you see when
a cap is used in a tesla coil).

What is going on here?  I know that these glass plates
can handle _at_least_ 22 KV, since these plates were
previously part of another glass/aluminum foil capacitor
that I had made, and I routinely charged it up to 22 KV
(DC).

About the only reason that I can come up with for the
destruction of the glass is that the glass plates
were so dissapointed about not being part of the
capacitor itself that they spontaneously committed
suicide.  It can't have been gremlins - I just had
the appartment sprayed for them. :)

Ideas?

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                     Kevin D Christiansen
      User Interface Group - The University of Virginia
              "The best VR that pizza can buy" 
   kevin-at-virginia.edu    http://www.cs.virginia.edu/~kdc4n/

"The universe is composed of space, galaxies, and intergalactic
 dust.  Galaxies themselves are composed of space, stars, and
interstellar dust.  From the omnipresence of dust, we conclude
that nature abhors a vacuum and won't pick up a broom, either."
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