This unit was made at basically zero cost. The transformer was salvaged from an old oil furnace, the power cord was cut off a discarded appliance, and the wires laying around the shop. Have fun and be safe.
Step 1: Safety First - Extreme Electric Shock Hazard!
ALWAYS disconnect the power before working on it, or adjusting anything.
DO NOT touch the terminals or rods when the transformer is energized.DO NOT do anything stupid.
DO NOT put anything conductive (including you) close to the terminals or rods when the transformer is energized.
Transformers of this type do not vibrate, make noise, light up, or let you know they are energized in any way. Just because there is no arc does not mean the unit is safe. Always check to make sure it's unplugged before handling.
Step 2: I've Got The POWER!
The heart of this project is the high voltage power supply. There are two readily available sources for this neon sign transformers and oil furnace ignition transformers. I use the ignition transformers as they can be had for free or a few dollars. Call your local furnace shop, they may have units that were removed from service that are of no use to them. Transformers that are removed from furnaces can still have enough power for this project. These instructions are based on a furnace transformer. There are other instructables which cover the use of neon transformers, so I won't rehash that info.
Check the data tag to determine the output voltage. You need at least 10,000 volts. More is better. This tag shows that the transformer runs on 120 volts and puts out 10,000 volts, which is sufficient for our project.
Step 3: Power Connections
There should be two wires coming out of the transformer. These are the lines that will be connected to the 120 V line. It doesn't matter which one is hot or neutral. Connections can be made with crimp connectors, solder, or wire nuts.
Step 4: High Voltage Connections
The high voltage terminals are usually a threaded stud or a smooth post. If it's a threaded stud, it will be easier to connect the wire. For the wires, you want something that's stiff enough to stand up on it's own and not flop around, but still be able to bend it into shapes.
TIG welding rod, thin brass rod, or even a wire coat hanger will work for this. Really, any conductive material will work. If using a coat hanger, it can't have any paint or coating on it. A plastic coat hanger won't work for this. If you didn't know that, maybe you shouldn't be playing with electricity.
In this example, a piece of electrical tape holds the tops of the wires at a set distance. This isn't necessary, but it keeps the wires from bouncing around. I don't know why, but as it runs, the wires will start moving. Maybe it's some sort of induced magnetism or something.
Step 7: Gee Mr. Wizard, Why Does It Do That?
Well, I'm glad you asked. Electricity follows the path of least resistance, which is why it starts at the bottom where the wires are closest. The arc is very hot, so it heats the air above it, make it more conductive. The arc moves up to that more conductive area, heating the air above that. It does this continuously until it gets to the point where the arc to just too long. At this point, it starts over at the bottom.
If you're going to have this on display, you should build an enclosure to keep people from touching it. I didn't go into how to build an enclosure, you can figure that out yourself and write your own instructable about it.