Salt Water Caps
Subject: Salt Water Caps
From: richard.quick-at-slug-dot-org (Richard Quick)
Date: Sat, 9 Dec 1995 03:44:00 GMT
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Quoting Ryan Ruel <rmr-at-christa.unh.edu>:
> I have completely finished my coil except for the capacitor.
> I'm planning to build some rolled polyethylene caps as soon as
> I gather the required materials and find the time. Until then,
> I would like to easily construct a salt-water capacitor so I
> can play with the coil until I build a better unit. I was not
> really able to find anything online about salt-water caps, but
> I keep seeing references to them. What would be required to
> build one capable of withstanding a 15 kv, 60 ma neon system?
> The secondary is 24 inches, 6 inches diameter, 880 turns 22 awg
> magnet wire. Primary is 14 turns, conical spiral, 1/4 inch
> copper tubing. Spark gap is a 6 inch, 16 electrode disk rotary
> system. Any advice would be greatly appreciated!
Before I jump into salt water capacitors I would note that the
rotary gap is overkill on a neon power supply. The rotary may
even cause a premature failure in a neon. This is not to say the
gap won't work, it will, I just prefer a decent static gap when
using internally limited (read neon) transformers...
Now, on to the subject at hand, salt water capacitors:
Tesla used salt water capacitors in the primary circuit of the
industrial sized Colorado Springs coil. He took ceramic standoff
insulators and placed them on wooden planking layed down over the
wooden floor. On top of the insulators he placed another layer of
wooden planking. This formed an insulated raised platform.
On top of the raised platform Tesla placed seven square metal
tanks. The tanks were made of tin or galvinized sheet steel and
had soldered seams to prevent leaks. Each tank was about three
feet square and two feet high. An excellent photo of these tanks
appears on page 324 of the Colorado Springs Notes.
Inside of the tanks, Tesla placed a number of glass mineral water
bottles. Using a saturated solution of rock salt and water
(brine), Tesla filled the bottles about 2/3 of the way to the top
and inserted an electrode into each bottle so that the conductor
was in contact with the brine. Each electrode extended out of the
top of the bottle. The bottles in each tank were grouped into
three equal parts or sections. Each group had their electrodes
wired together to form a high current buss.
Once the bottles in a tank were filled and connected, brine was
carefully poured into the tank until the level of salt water in
the tank and the level of salt water in the bottles were the
same. This is a salt water capacitor. The brine in the tank forms
one of the two capacitor plates. The glass bottles are the
dielectric. The brine inside of the glass bottles forms the
second plate of the capacitor.
Tesla had some simple covers that fit over the tops of the tanks.
There were three holes in each cover. A high voltage buss cable
passed through a porcelain insulator placed in each hole. Each
cable represented 1/3 of the capacitance of a tank.
Salt water capacitors of this type are messy to build, difficult
to maintain, and very lossy; but they do work. In order to
supress corona losses, prevent flashovers up and down the sides
of the bottles, reduce churning of the liquid plates during pulse
operation, and prevent evaporation of the brine plates; both the
bottles and the tank were topped off with several inches of heavy
For today's coiler a smaller and somewhat simpler salt water
capacitor may be easily and cheaply constructed for use until a
suitable plastic film capacitor can be constructed/purchased. The
most common is the beer bottle/brine cap. I have heard all of the
jokes about these... That you don't really need the brine, just
get a case of cheap long-necks and let nature take it's course:-)
But anyway, you do need about a case of long neck bottles (clean
with labels removed) and beer bottles do work very well. You will
also need the twist off caps. Buy a standard Rubbermaid plastic
dishpan (11-1/2 x 13-1/2 x 5-1/4 inches, Wal-Mart under $7.00).
Shop around at a hardware store(s) and buy twenty-four 1/4 inch
diam. by 6 inch long galvinized steel carriage bolts. You will
also need seventy-two, 1/4 inch thread, nuts that will thread
onto the carriage bolts. You will need a few quarts of the
cheapest grade, straight 30W, non-detergent, motor oil; don't
spend money for multi-viscosity high detergent oil; you can use
the best mineral oil in a pinch, but it works no better than
cheapest straight grade engine oil. You will need some heavy duty
aluminum foil, and you may want to get a large tube of clear
silicone sealant. Get a 1/4 inch diam. by 1 inch long brass screw
and two nuts.
Using a hole punch, punch a 1/4 inch hole into the center of each
twist-top bottle cap. If you punch from the top of the cap down
it will bend, so flip the caps over and punch them through from
the bottom. Run a nut down the threaded shank of the carriage
bolt about an inch, slide a bottle cap over the threaded shank,
then run another nut down and tighten. This should produce an
electrode that will drop down inside of the long neck bottle and
will secure with a twist on the bottle cap. After fitting and
adjusting, you may want to remove the top nut and the cap. By
bedding the cap to the electrode with silicone sealant you can
prevent leaks and stop the nuts from loosening.
After completing the electrode assemblies for all of the bottles,
mix up a few gallons of warm saturated brine in a five gallon
plastic bucket. I use the cheapest grade of rock-salt which is
readily available here in the winter season. In many parts of the
country water softener salt is available year around. Fill each
bottle with brine to a depth of about 4-3/4 inches, insert the
electrode part way, then top the bottle off with a couple of
inches of motor oil. Use a small funnel or "squirt bottle" here
to prevent sloshing and do not overfill with the brine or oil.
Now if things are going right you have a bottle with the elect-
rode assembly inside and held over to the side. Before fully
inserting the electrode assembly and cinching down on the cap you
can smear some silicone sealant around the threads on the cap and
around the top of the bottle. This will prevent leaks. Cinch down
firmly on the cap.
Fold a very large sheet of aluminum foil over double and scrunch
it down into the bottom and sides of the plastic dishpan. Some
dishpans have a hole molded into the lip, if yours doesn't then
cut or drill one. Get a 1/4 inch diam. by 1 inch long brass screw
and two nuts. Roll up a bit of the foil at the hole end of the
dishpan, punch a hole through the foil, and make a terminal by
passing the brass screw through the plastic lip and foil. Secure
with one nut. The second nut is for securing the connection to
the tank circuit.
Set the bottles into the dishpan. You will find that only 16 of
the bottles will comfortably fit, but you will most likely need
the other eight bottles and a second dishpan to get a .012 MFD
capacitor. One dishpan with 16 bottles gives about .008 MFD.
Wire the tops of the bottles together with some stout copper
cable, strap, or copper tubing that is pounded flat and drilled
for the 1/4 inch carriage bolt electrodes. You can use some
strips of aluminum flashing with holes punched through if you
smear a little electrical corrosion inhibitor on the connection.
When everything is set and settled down, fill the dishpan up with
brine to within a 1/2 inch or less of the rim. Pour a thin layer
of oil over the brine to suppress corona. With the dishpan this
full it is not fair to say this capacitor is portable. It will
slosh over if moved. It is probably best to fill the pan right
where you intend to use it.
There are as many variations on these capacitors as there are
coilers building them. Heavy plastic bottles or buckets give a
much higher Q factor, but the dielectric constant ("K") of the
plastic is a low ~2 compared to the much higher K of bottle glass
which comes in ~7. This means the capacitors must get physically
larger. I have seen a pretty nice vertical stack brine capacitor
made from five gallon plastic buckets. A strip of stainless steel
strap was hung across the bottom of the bucket, up the side, and
over the rim. About 1.5 quarts of brine was poured in. Another
bucket was firmly seated inside of the first, another strap,
more brine, another bucket, etc. The guy was processing 8 KVA
from a pole pig through this corona glowing ozone producing
monstrosity. Like I said, these are messy, inefficent, high
maintanence... but they are very cheap and they do work.
... If all else fails... Throw another megavolt across it!
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