'suicidal' glass plates

To: mail11: ;
Subject: 'suicidal' glass plates
From: pierson-at-msd26.enet.dec-dot-com
Date: Sun, 29 Sep 96 14:07:11 EDT
Cc: pierson-at-msd26.enet.dec-dot-com
>From: "Kevin D. Christiansen" <kdc4n-at-cs.virginia.edu>
>Subject: Suicidal glass plates


>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
>here 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).

>Ideas?

	Speculation...

	Voltage greater than 22kv was present???

	All i can think of is transients from the rectifier.  (Rectifiers
	accumulate bits of stored charge on each half cycke, then dump it,
	leading to some nonobvious HV spikes when they switch.  I ASSume there
	was a stray (capacitive) path that MADE the glass plate part of the
	capacitor (leakage from HV supply to floor, etc, etc...)

	regards
	dwp

(Measuring grounds:
	Good news.  Found the book.
	Bad news.	Nothing easily usable.  (its the one i quoted from
	about 6 mos ago)
	Good news:	Its got a bibliography, which i will post when i
	remember to bring the thing in...)

Possibility:
	Its by Joslyn, Inc.  They MIGHT have a WWW site up....)