Salt Water caps
Thought I would drop a line about salt water caps. These types of capacitors
are ideal for beginners to Tesla coiling to begin with.
I first decided to make a Tesla coil powered by a 15 kV 30 mA neon about 8
months ago after "listening" to Richard Quick discussing Tesla coils over on
fidonet. At the time he was discussing his plastic capacitors but from the
construction details and the very-hard-to-get-over-here thick polyethylene
sheet, I knew that it would be a major construction exercise. I asked him if
there were any alternatives and he and others told me about salt water caps.
I bought a plastic box ( about 60 cm (24 inches) by about 50 cm (21 inches)
and about 40 cm tall). I then got a sheet of aluminium foil and glued it onto
the bottom of the box and up one side of the box. To stop any arcing between
the aluminium up the side of the box, and the top of the beer bottles, I
placed a sheet of window glass in front of the aluminium foiled side of the
box and secured it with a few cable ties linked together. The plastic box I
had bought had a lip about 1 inch wide. In this lip I drilled a hole and
placed a large bolt,nut and washer. I scrunched up the aluminium foil and
secured it under the nut and bolt. The nut and bolt formed one terminal of
I then got a crate of Lion Red (believe me- this stuff is heaps better than
that Bud that you lot drink over there) and proceeded to thoroughly clean the
bottles (750 ml) in detergent. I let the bottles soak to remove the labels.
While the bottles were soaking I made up a saturated solution of salt
solution in a bucket.
When the bottles had thouroughly dried I filled them slightly less than 3/4
full with the salt solution and placed them in the box. When the box was full
with bottles (about14 bottles I think) I filled up the box with salt water
until the water mark in the bottle was at the same level as the salt water in
the bottles. To complete the capacitor I ran strap into each bottle and had
the other terminal of the capacitor on the side opposite the first terminal.
I also put a bit of motor oil into the salt water in each bottle and into the
main tank to suppress corona. Finally I draped a plastic sheet over the
completed tank to provide protection should one of the bottles rupture
explosively though I dont think this is very likely.
This capacitor ran my first ever neon powered Tesla coil. I still remember
being really surprised by how loud the spark gaps sounded when I first
powered up the coil (if you want to know how surprised I was, just ask my dry
It is only a small coil (secondary approx 2.5 inches in diameter) and about
21 inches high but it gives out sparks about 9 - 10 inches long. I understand
that I am lucky to get sparks so long on my first attempt at making a neon
powered coil- most people reckon that on their first coil that their spark
length is only about 4 inches long. I guess I must have fluked a good setup.
When the coil is sparking and you look into the cap tank you can see a fair
bit of agitation of the salt water in the box and you can see lots of corona
around the bottles in the tank. I guess the agitation in the tank may be
caused by corona wind. The type of cap I have described is only really
suitable for running small coils as its capacitance is quite low. But for
beginners (such as myself) it is the ideal capacitor (very easy and quick to
make). To get bigger capacitance you need more bottles and the size of the
capacitor becomes unwieldy very quickly. Also glass is not an ideal
dielectric as it is very lossy. This means the cap will not discharge all its
energy into the coil as some is lost in heating the dielectric etc. This
results in purple, spindly sparks from your coil instead of the brilliant
white hot sparks that you see from coils using plastic dielectrics.
Again on the advice of Richard Quick I tried to make a bigger salt water cap
by using stacked plastic buckets filled with salt water. I secured each
bucket in place by using a cable tie on each side of the bucket through the
lip of each bucket and tying them through the lip of the nesting plastic
bucket. I then filled the spaces between the buckets up and connected the
buckets together using strips of aluminium foil. Unfortunately I was never
able to get this capacitor working properly- I kept getting sparking between
the buckets. I tried lowering the salt water level in each bucket but still
no joy. I think the surface of the plastic buckets may not have been
perfectly dry or clean. In the end I gave up on the stacked bucket capacitor
as I never could get it to work.
At the moment I am building a 6 inch coil and I will be making a flat plate
cap using 250 micron plastic film between aluminium foil plates.
_/_/_/ _/_/_/_/ Mark Conway
_/ _/ _/ Deep Thought BBS, Auckland, New Zealand
_/ _/ _/ A FirstClass(tm) Macintosh GUI BBS
_/_/_/ _/ Internet: mconway-at-deepthnk.kiwi.gen.nz