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Max et cap
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From: Payne, Will E [SMTP:will.e.payne-at-lmco-dot-com]
Sent: Monday, August 24, 1998 7:31 PM
To: 'Tesla List'
Cc: 'Kip'
Subject: FW: Max et cap
My recent query on use of plastic soda pop bottles for SW caps reaped two
sets of responses. Some folks warned me about their bad experiences with
soda pop bottle SW caps, which perforate, arc thru and dribble SW on the
floor. However, Mike Foster is a fount of useful knowledge on sucessful use
of PET dielectric bottles at very high DC voltages.
I am inclined to experiment, perhaps even discover a cure for whatever
ailments have plagued previous attempts. The list provided sevral stories
of past failures which I take very seriously, so I will be especially
cautious of AC heating, corona and hot spots attacking the dielectric. Mike
is right, I probably could have made the foil tighter, so I will try again
with liquid electrolyte inside and out, which may also help prevent corona
due to sharp points. Did anyone who tried soda pop bottles relate their
difficulties to this ?
Although a firm beleiver in theory, I believe it must be tempered with
practice. Thanks to Mike and to all who sent me their experiences with soda
pop SW caps.
<snip>
> ----------
> From: Michael Foster[SMTP:michael.foster-at-mailexcite-dot-com]
> Sent: Monday, August 24, 1998 19:52
> To: Payne, Will E
> Subject: Max et cap
>
> My, you are a quick reply! You must think and type a
> lot faster than I do. I found three books at Amazon by
> Maxwell and I guess I'll just order them and begin to
> wade.
>
> If your 2-liter Coke bottle only had a capacitance of a
> little over 2nf, then the bottle walls must be thicker
> than the mineral water bottles, or you are not as good
> as I am at applying the foil. Pop bottles have to
> withstand 6 atmospheres of pressure because of the
> carbonation, so they might be thicker than mineral
> water bottles. Why don't you try immersing the pop
> bottle in a larger container of salt water and see the
> difference between the foil wrap and the immersion
> type? The pop bottles and mineral water bottles are
> both made of the same stuff, polyethylene terphthalate.
> PET has a dielectric constant of about 3.2. Both the
> dielectric constant and the dielectric strength vary
> a bit with the average molecular weight of the polymer
> and with the stress of molding, or how far the plastic
> is stretched when it is blown into the mold. Similar
> effects are taken into consideration when rating
> commercial metallized polyester caps as the film used
> is the same material as the pop bottles. In this case,
> the film is "biaxially oriented" meaning that it is
> stretched in two directions while it is hot. This
> increases both the mechanical and dielectric strength,
> which are really the same thing. I find that it is
> important to have the salt water at near saturation to
> work really well. In fact, one of my favorite tech-
> niques is to saturate the solution with both salt and
> Epsom salts (magnesium sulfate.)
>
> I have not tried any of these capacitors in a Tesla
> coil, so all of my theorizing is just a bunch of words
> strung together until I do. All of my experiments have
> been with DC, but the voltages have been very high in
> some cases. Just to see what would happen, I made a
> salt water cap which was two cells separated by PET
> film only .00048 inch thick (12 microns.) And no,
> that's not an extra zero. Anyway, I charged this up to
> 25kv repeatedly with no trouble at all.
>
> The reason I bring up all this stuff about the stress
> and orientation of PET, is that if you heat up a spot
> on this plastic, it tends shrink there and get a little
> thicker. I just thought that with the addition of
> water cooling on both sides of the polyester, this
> might constitute a "self-healing" dielectric.
>
> Have any of these people who have warned against trying
> this actually used this type of capacitor in a Tesla
> coil or are they just a bunch of hand-wringing
> theorizers like me. I guess I'm just going to finally
> break down and build a coil myself to test it out. And
> by the way, I will cut apart some 2-liter Coke bottles
> and measure the wall thickness with a micrometer, which
> is how I know the thickness of the mineral water
> bottles. They are all amazingly consistent.
>
> Yours truly,
>
> Michael S. Foster
>
<snip>
Will