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Saltwater Cap How-To
Original poster: "Christopher Boden by way of Terry Fritz <twftesla-at-qwest-dot-net>" <chrisboden-at-hotmail-dot-com>
Here's the finished rewrite on the Geek Group Bucket Caps
Design and construction of The Geek Group Bucket Capacitor
By Chris Boden 5-2002
Over 100 years ago Tesla himself used champange bottles to make the
capacitors he used in the tank circuits for his coils. The basic design of
the saltwater cap has changed little since then. They are lossy, bulky, and
delicate, but they are the cheapest way to build a reliable cap for Tesla
coils. Because of this it is the only type of cap availible for many
beginning coilers. The Geek Group, in the late 90ís decided to design a
better system for building these caps and came up with a design that is
lightweight, low-cost, safe, and reliable. We have been using these caps for
several years with great results. Several others have built them, and they
have proven to be a reliable means for entry-level coilers to build caps for
Tesla Coil use.
12 Longneck Corona brand Beer bottles.
1 5-Gallon white HDPE plastic bucket with an airtight lid.
1 small rubber stopper (optional)
2 5-way binding posts (get these from Radio Shack)
5 cans of Mortons Iodized Salt (any table salt will work, youíll need a lot)
Detergent-free motor oil, or Mineral oil. DO NOT USE VEGETABLE OIL!
14AWG or larger solid, bare copper wire.
You may substitute parts, but they may result in poor performance, different
values, etc. The design has been tested with several different bottles and
we have found Corona to be best.
Empty the bottles (this is the fun part). Then wash them throughly and let
them dry. Make sure to rinse any soap residue from them. Make sure the
bucket is new and clean, free of cracks and pinholes. We use self-sealing
buckets that swimming-pool Chlorine pellets are packaged in. Itís important
that the bucket has an airtight lid, yet can be removed for repairs as
bottles will, over time, fail and require replacement.
Place the 12 bottles into the botom of the bucket. They should fit well
enough so that they will slosh a little, but none will be able to tip over.
Get a 2-quart pitcher and dump an entire can of salt into the bottom (about
one pound). Then fill it about ĺ or so with HOT (not boiling, but hot)
water. Stir this until it stops absorbing salt, you will still have salt on
the bottom, if you donít, add more until it stops absorbing it and it starts
to collect on the bottom. Itís impossible to add too much salt.
Let the mixture sit for 2 minutes, allowing the salt to settle, then fill
your first bottle to the base of the neck. You can remove them from the
bucket to fill them if it makes it easier. DO NO FILL THE BOTTLES TO THE
TOP, only to the very base of the neck, where the bottle starts to get
narrow. Fill the remaining bottles as well. Make more saltwater as needed.
Step 5: With all the bottles in the bucket, fill the space around the
bottles with saltwater to the same level as the saltwater inside the bottles
(the base of the necks). You should now have 12 bottles filled to the base
of the necks, sitting in a bath of saltwater to the same level.
Pour about a tablespoon of salt into each bottle, then sprinkle about a
half-cup throughout the bath area around the bottles. This will ensure that
the water is always super-saturated. The small dusting of salt on the bottom
will have no effect on the operation of the cap.
Pour a layer of new motor oil, or Mineral oil on top of the saltwater. Pour
it slowly to avoid bubbles. Fill the bottles to the top with oil, and pour a
layer about 1Ē thick on top of the bath. This prevents evaporation and
corona buildup, it also drasticly improves the insulation qualities of the
cap. DO NOT USE VEGETABLE OIL since it will in time become rancid. Opening a
cap to have to change a bottle full of year-old rancid oil is not a fun
Bend the wire into a U shapes. These will become the jumpers for the cap.
They need to reach down inside the bottles, to a point about an inch from
the bottom. Each bottle needs to connect to at least 2 other bottles so that
they are all wired in paralell.
Drill a hole in the side of the bucket (above the full level) and place a
Binding Post connector there. Connect a wire from the binding post to hang
along the inside of the bucket down into the saltwater, about an inch from
Install a binding post in the lid of the bucket. When the lid is installed
there will need to be a wire connecting the binding post to the connecting
wires on all the bottles. Do not let any bottle wires or the top connection
come within 3 inches of the side terminal. The wire on the side should not
be in contact with anything above or inside the bottles. Itís ok if I
touches the sides of he bottles though.
Step 11: (Optional)
Drill a hole slightly smaller than your rubber stopper and install I in the
lid. This is a safety vent in case the cap builds up internal pressure from
overheating. We have this on a couple of our caps and have never had one
pop. But if it makes you feel better, install one.
Step 12: Place a light coat of oil around the seal, and intall the lid on
your bucket after connecting the wire. Congratulations, you now own a
Saltwater Capacitor! Put a meter on it and it will measure about .0125uF
perfect for a 15/60 NST. The value of the cap will take a week to settle as
the oil absorbs a small amount of the water, but it will not affect
The finished cap will be rated about .0125uF -at- 40kV.
Rules for safe handling of your new cap.
1. NEVER TIP YOUR BUCKET! Anything more than 35* or so is asking for
2. Donít drop it, remember, thereís a lot of glass in there.
3. It IS ok to leave them outside in Winter, the water will not freeze so
the bottles wonít crack.
Christopher "Duck" Boden Geek#1
President / C.E.O. / Alpha Geek
The Geek Group
Because the Geek shall inherit the Earth!