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Re: Plastic Pop Bottle SW Caps



Date: Wed, 26 Aug 1998 18:48:19 -0700
From: Jim Lux <James.P.Lux-at-jpl.nasa.gov>

Tesla List wrote:
> 
> Last night I did a few experiments with a soda pop bottle PSWC (Plastic Salt
> Water Capacitor) .  The wall of a 2l plastic coke bottle measured 0.010 +-
> 0.0005 inch wall thickness.  With a foil wrapping the bottle had measured
> 2.4 nF, I fully expected to get one or two  nF more with electrolyte
> replacing the outer foil, but to my surprise the capacitance increased to
> 8.5 nF !!  Mike you were right about my skill at the subtle art of
> bottle-foiling !
> 


I'll jump in here with some comments...

I would think the whole idea of using salt water is that it provides the
conductor, and that adding aluminum foil to the combination is only
asking for trouble. Of course, aluminum foil in salt water is a recipe
for corrosion in the first place, and the aluminum salts that would
deposite out are probably a good insulators as well.

The thing to do, then, would be to get a big metallic corrosion
resistant tub, fill it with saturated salt solution (if you use a copper
tub, use Copper Sulfate). Then put your bottles, half filled with more
salt solution in to the tub, spaced appropriately (several inches, I
should think) from the sides and bottom (ideally, use an insulating
support on the top of the bottles (out of the liquid)) Then, drop your
bare copper wire (assuming copper sulfate here) into the liquid inside
the bottles, but not touching the plastic.

The saturated salt solution provides a uniform contact over the entire
area of the dielectric (without any field concentrations due to wrinkles
in foil, sharp corners, etc.)  Of course, I suspect that most bottles
are hardly uniform in thickness, so the field will concentrate at the
thin spots. Based on this, bottles designed to hold pressure would be
best, since thickness nonuniformities would make them weaker for the
same amount of material. (Time to go drink that case of Champagne, all
in the name of Science, right)....

And, of course, floating a layer of oil over the top of the liquid, both
inside and out, would reduce corona discharge at the edges.

I don't know about the relative conductivities of various salts. If I
had to start, I would choose a salt that would have high solubility (so
there are lots of ions in the solution) and would be chemically similar
to the electrodes (so it doesn't electrolyze or electroplate). Copper
sulfate is pretty soluble, and is copper compatible, but I don't know
how it compares to sodium chloride or magnesium sulfate. I also don't
know if choosing a salt that has di or trivalent ions (like Mg++ or
Al+++) would improve the conductivity over a monovalent ion (like Na+).
It might: you have to move half as many ions for the same number of
coulombs.  Toxicity might also be an issue. Sodium Chloride in water,
while cheap and readily available, does electrolyze to Chlorine gas and
Sodium Hydroxide (which can, in turn, be turned in to Sodium
Hypochlorite: bleach, a process done on a gargantuan scale
commercially).

What about using an electrolyte that isn't a water solution. Perhaps it
is a bit exotic, but sodium is a fairly good conductor, and melts at a
fairly low temperature (97.5 C). Mercury is a good conductor, a liquid,
but does have significant safety hazards. Gallium is being used as a
mercury replacement, and melts at low temperatures (it'll melt in your
hand). Some ionic salts melt at low temperatures, and might be decent
conductors when molten.

Be original, be exotic, be the first on your block (or in the world) to
use a sodium/sulfur/ceramic capacitor at 300 degrees C.

Faugh on those who would use centuries old technology like glass leyden
jars or ordinary salt water.

That is, unless you are building a historical replica, in which case the
hand turned oak balls gilded with gold leaf, the gutta percha insulation
(carefully cured over a fire in the Amazon jungle for the optimum carbon
loading to minimize corona), the silk insulation overwrap, and so forth
are absolutely necessary.