This is the part of the Tesla Coil that will perform the first step of voltage amplification.

For this section, you will need the following items:
1. A 6,000 volt, 10mA transformer (0.01 Amps), though you can use one rated up to about 10,000 volts. If you get something over 10mA at 10KV though, it might be overpowered for a coil of this small scale.

The other part of this section consists of the capacitor. There are a few options here. You can either buy them, or make your own.

All Electronics once had 10KV .001uf capacitors, but they're sold out now; I did find some alternatives:
Hosfelt Electronics has caps with the same ratings. They're $0.99 each, part number 15-841.

You may need to hook capacitors up in series and parallel, so a bank of them would look like this.
A bit about what happens to capacitors when they're rewired:
When you hook them up in parallel, the voltage of the final bank is the same as that of the lowest capacitor in the bank. Since these are all going to be the same thing, it's just the voltage of any capacitor present.
When you hook them up in series, the voltage is additive, but the capacitance decreases like this:
1 / Ctotal = 1 / C1 + 1 / C2 + 1 / C3

So you'd get something like this, using the 9KV, .002uf caps:
1.) 1/Ctotal = 1/.002 + 1/.002 + 1/.002
2.) 1/Ctotal = 1500
3.) Ctotal = 0.0006...uf

So here, you wind up with a capacitor bank with a rating of 27KV, at .0006uf.
Take two banks of these, and hook them in parallel, and you get a bank that'll give you 27KV, at .0013uf. That will be sufficient capacitance, though you could add another 27KV, at .0006uf bank if you choose, for .002uF capacitance. This KV rating will be good for a transformer up to 9KV. I'd recommend leaving your capacitor bank value 2-3 times the transformer's rating, but try for closer to 3x. Doing this will help ensure that the capacitors aren't damaged by surges in the circuit.

The type of capacitor I first used was a salt water capacitor. They are relatively inexpensive and easy to make, however they only hold up to about 2 minutes of run time. A little after this time, the sparks visible around the foil weaken the plastic. After a while, the plastic is punctured by one of these sparks. The capacitor is now irreparably damaged. Get rid of the oil and salt water, recycle the bottle after removing the aluminum, but save the cap and bolt assembly. These are still useable, so you will not have to make more for future salt water capacitors.
To build two salt water capacitors, you will need the following items:
1. Two 1-liter plastic soda bottles with the lids.
2. A few pounds of salt, preferably without iodine.
3. The roll of aluminum tape that was used on the secondary coil.
4. Clear, strong tape.
5. About 3/4 cups of motor oil. Any type works.
6. Silicone sealant.
7. Two 8-inch long by 1/4" bolts with 6 matching hex nuts.

Wrap each bottle in 1 layer of aluminum tape. It should start about 1" from the bottom of the bottle, and go up until the the wall of the bottle begins to curve. Everything in between these two ends must be covered with aluminum. It is OK if the tape overlaps in some places, but try to keep this to a minimum. Once it is wrapped, smooth the foil down with a spoon. Now wrap it tightly with the clear tape. The tape should start about 1/2" away from the end of the foil, and cover all of the bottle. Before you start this, make sure to keep an area of 1 square inch of foil exposed toward the bottom of the bottle. This section will be used to connect the foil sections of the two capacitors together.
Drill a 1/4" wide hole in the center of each of the caps of the 1-liter bottles. Screw a nut about 1" down each of the carriage bolts. Put each bolt through a cap, so that the end, not the head, would stick out of the top of the bottle. Leave about 3/4" of the bolt above the top of the cap. Screw another nut down onto the end of the bolt, so that both hex nuts clamp down on the cap, holding it tightly. Now apply silicone sealant to both sides of the cap, forming a watertight seal. Keep the sealant away from the threads on the inside of the cap. The cap will need to be screwed onto the 1-liter bottles later on. Also keep the sealant off the top of the nut on the top of the cap. Only apply it to the area where the nut contacts the cap. Allow the sealant to dry.
Now mix a solution of salt water. Pour two liters of water into a pot or bowl, and pour 1 cup of salt into this. Stir until the salt is completely dissolved. Now pour it into each of the bottles, leaving 1/2" space at the top of the bottle. Fill most of this space with motor oil. Now screw one of the cap and bolt assemblies into each of the bottles. Some oil will initially stick to the bolt as you lower it into the bottle. This is normal, and it will later float to the top.
Attach the protruding ends of the bolts to each other using 8 gauge wire, and some ring connectors. Attach a 1 1/2 foot piece of wire to a ring connector, and attach this to one of the bottles, and well as the other connector. Secure each connector down with another hex nut. Cover this connection with silicone sealant as well.
Now strip the insulation off both ends of an 8" long piece of 8 gauge wire. Do the same for a 1 1/2 foot piece of the same type of wire. Bend the bare wire at 90o so that the entire piece has somewhat of a U-shape to it. (It should look something like a drawer handle.) Do the same for one end of the 1.5 foot piece as well. Attach these to the bare sections of foil on each capacitor using tape. Attach the 1 1/2 foot piece of wire to the capacitor with the other long piece of wire attached to it. Make sure to cover the bare section of foil entirely. Put hot glue around each of these connections to help hold the wire in place.

This is what the capacitors will look like.

Cost of the Salt Water Capacitors
1. The 8" bolts cost $.40 each, and the nuts for them can vary from $.05 each to $.15 each.
2. I used clear postal tape. $2.00.

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