Re: Glass Caps

Hi Nick, All,
    I did experiments with glass plate capacitors 25 years ago in high
school.  My first capacitor consisted of one eighth inch window glass
with aluminum foil held down with electrical tape.  It lasted for
about 6 few minute runs and then shattered and arced through taking
the 12 kV, 60 mA neon transformer with it.  I took the transformer
back to the neon sign shop near the dry wash gully ( I lived in
Palmdale, California at the time).  The man at the shop pointed to a
conical pile of neon transformers five feet high and said that I could
have anyone of those.  I picked out a new looking 12,60 near the
bottom of the pile, in the center, and continued experimenting.  I
hypothesized that the heating and eventual failure of the glass plates
was caused by the corona, partial discharges from internal voids, and
the friction from the foil buzzing at sixty hertz.   I found that if
the metal plates were thermally conductive and massive enough, with
nice rounded and polished edges, then the glass would survive if it
was at least one fourth inch thick.  I made my last glass capacitor
with galvanized steel sheets and one fourth inch thick glass plates.
It still buzzed a little but never broke.  My last capacitor consisted
of an eighteen inch square by three eights inch thick piece of blue
acrylic sheet from an old window in a land fill two miles out in the
Mojave desert ( I was attacked by 6 wild dogs but got away by
reflecting the sun off of the acrylic sheet at them).  I got 14 inch
square by one eighth inch thick aluminum plates from the metal shop
teacher.  I filed and polished them by hand.  This made a great
capacitor.  The corona was much reduced also.  I had to keep the runs
down to less than a few minutes to keep the acrylic sheet from
internally overheating.

    If you put them in oil vertically so that the oil can flow and
dissipate the heat then they should be fairly rugged, by inference,
though I never tried this.  Check the glass edges for very fine
cracks.  Check the whole plate for very small bubbles.  These are the
places where the heat will build up and eventually break the glass
from partial discharges, etc (mine broke from a void in the center).
Try not to get a piece of glass with these.  Also, ask the glass
person if they can flame polish the edges.  This helps a lot.

    I have always tried to design high voltage apparatus for the open
air environment.  Oil is heavy and requires periodic maintenance.  In
my two million Volt Marx design I used one eighth inch thick aluminum
conductors with polished edges for the capacitor interconnecting
conductors.  There are 19 stages.  Each stage has two conducting
plates on a lexan surface separated by 10 inches (10 inch creepage
path).  One plate is charged to 60 kV while the other is charged
to -60 kV.  That is 120 kV total across a ten inch surface times 19
plates for up to 3 minutes charge time.  The total leakage is less
than 20 microamperes for the whole machine.   To paraphrase Richard
Hull +ACI-It's all done with field control+ACI-.

-----Original Message-----
From: Tesla List +ADw-tesla+AEA-pupman-dot-com+AD4-
To: tesla+AEA-pupman-dot-com +ADw-tesla+AEA-pupman-dot-com+AD4-
Date: Thursday, March 11, 1999 4:18 AM
Subject: Glass Caps

Original Poster: NickandSim+AEA-aol-dot-com

To All,
          I have just built my first glass plate cap.  I have been
told that
they are fine running in air.  Is this true or should I oil bath it?

Thanks in advance
Nick Field