Secondary Coil

This is the part of the Tesla Coil that people usually remember seeing. It is a coil of wire wrapped around a tube. The sparks come from the top of this.

For this section, you will need the following items:
1. A 12 inch length of a 3 1/2" diameter cardboard tube; the width of the walls of the tube should be 5/16" or thinner.
2. Some sort of plastic cap that can be used to close the cardboard tube temporarily.
3. One quart of polyurethane varnish, and some 1-inch wide paintbrushes. The varnish MUST NOT be water based. It has to be polyurethane or oil based. (paint thinner is very useful for cleaning the brushes). Fiberglass resin may be used instead of varnish. It is thicker and will require fewer coats.
4. About 700 continuous feet of 24 to 26 gauge enameled magnet wire on a plastic reel. This will weigh a little under a pound. The reel should have a hole through the center of it. This will come in handy for use with the winding rig explained below.
5. Two plastic discs that can be used to close each end of the tube. They must be 3 1/2" in diameter, and about 1/4" thick.
6. Two rubber bands that will fit tightly around the tube.
7. A wooden wheel about 2 inches wide. It should have a hole through the middle. You will also need a small roll of aluminum tape.
8. A 6-inch section of a wooden dowel. The dowel must fit snugly into the hole in the wooden wheel.

Coat the outside of the tube with polyurethane varnish. I coated the outside 7 times with varnish, allowing it to dry between coats. The inside must also be varnished. To do this, I capped one end and simply poured varnish into the tube. When you do this, make sure that the cap is as water-tight as possible. I lined the edges of it with duct tape, and this worked very well. Spread newspaper before doing this, in case some varnish does leak out. I turned the tube slowly to cause the varnish to coat the ENTIRE inside of the tube. Leave the tube dry thoroughly. It is critical that the tube is nearly saturated with varnish.

When you are not busy varnishing the tube, you can begin construction of a simple winding device.
You will need the following for the winding rig:
1. A cardboard box about 1 1/2' x 1/2' x 2'.
2. A wooden dowel that will fit through the hole in the reel of wire. It should be thicker than 1/4", and less than 3 feet long.
To assemble it, push a dowel through one side of a box. Then push it through the middle of the reel of wire, and then through the other side of the box. The front of the box should be cut out. This configuration allows the reel to spin freely.

You can then begin winding with the 24 gauge wire. The winding should start about 1/2" inch from the end of the tube, and should be parallel to the end. Leave a foot of wire before you start winding. When you start winding, use some hot glue to secure the first turn in place. The turns should be wound right next to each other, as well as tight on the cardboard tube. As you wind, place a dab of hot glue every 15 to 25 turns. This is 'insurance' in case you slip and the wire loosens up on the coil. This way, the rest of the coil would not be affected by this. Make sure that the wire does not overlap.
Once the coil has been wound, the final turn can be secured in place by a few dabs of hot glue. What you should now have is a coil that is tightly wound with wire, all except for 1/2" on either end. It now needs to be coated with varnish. You can use the same varnish that you coated the cardboard tube with. Right before you are ready to varnish it, remove all of the dabs of hot glue, except for one or two that will hold the end turns in place. Do not coat the tube lengthwise, as this will form small air bubbles, which can greatly decrease the coil's performance, and possibly damage the secondary coil. You should coat the coil at least 20, yes I do mean twenty, times. This improves the coil's performance, and it keeps the turns of wire from loosening up. It also keeps out all moisture. If 20 coats of varnish is too much for you (who can blame you) I have heard that fiberglass resin is more efficient to apply. It is thicker and should only need about 3-5 coats.

This is what the coil should now look like.

Take one of the plastic discs, and drill a hole in the middle of one. The hole must be wide enough to allow the wooden dowel to fit in snugly.
Once the varnish has completely dried, put epoxy on the ends of the secondary coil tube, and put one plastic disc on each end. Make sure that there is no wire on the inside of the coil.
Put a few drops of Elmer's Glue into the hole in the middle of the wheel. Push the dowel through the wheel until it is flush with the other side of the wheel. Allow it to dry.
Cut off small strips of the aluminum tape. They should be around 1/4" wide by 1". Begin applying them to the wooden wheel until it is covered with tape. Also put tape on the first inch of the dowel from the wooden wheel. It then needs to be smoothed out. You can use something like a spoon for this. This assembly will serve as what is called the terminal capacitor or discharge terminal. The toroid shape works very well for this purpose.
Put a small amount of epoxy into the hole in the plastic disc on the end of the secondary coil. Push the bottom end of the toroid assembly into the hole.
Sand the enamel off the last 1/4" of the free wire on the end of the secondary with the toroid assembly attached to it. Air wind the wire in a few loose turns upwards to the toroid. When it contacts the bottom of the toroid, secure the wire in place with a piece of aluminum tape.

Cost of the Secondary Coil
1. I found the cardboard tube in a trash can at a machine shop's shipping department. You should also be able to get these inexpensively at office suppliers. The plastic cap may come with the tube.
2. The varnish costs about $8.00. The brushes cost me $1.00 each.
3. I was able to get my wire for $3.00 a pound in surplus. Unfortunately, it sold out in less than a week. Kelvin sells wire 24 gauge magnet wire for $9.50 a pound. Unfortunately, there is a $20.00 minimum order, plus a $6.00 shipping charge. The Electronic Goldmine occasionally gets ahold of batches of magnet wire, and their prices are very reasonable for the amount they sell on a roll.
4. The wooden dowel is about $.50. You can get a box from any retailer - all of stheir products arrive in boxes, which must be opened sometime, right?*
5. The plastic discs can be cut from a sheet of acrylic plastic. I had to buy one that measured 2 feet by 3 feet, and it cost $5.00. Maybe you can snag a small scrap of it for free from a store (ask first though).
6. The copper braid cost $2.00 from American Science and Surplus. However, if you would like to purchase this, AS&S does have a $10.00 minimum order, plus shipping.
7. The aluminum tape is available either in the tape section or the dryer vent and duct section of a hardware store. It costs about $3.50 for a small roll.

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