The Transformer
Neon sign and oil burner transformers are often used to construct Jacob's ladders. When buying a transformer, get something with a voltage rating of 9-15kV. The current output is not that important. Try calling around for used transformers, because they run *a lot* cheaper than buying them new.
Oil burner transformers can be found at places that do furnace repair and various mail order catalogues. Sometimes, furnace repair companies have old units that they'd be willing to give away for next to nothing. New, they run about $30-$40.
Neon sign transformers (NSTs)can be found at just about any place that makes neon signs. Check your local yellow pages. New, they run anywhere from $75 to $150, so a used one is ideal. They often have old transformers lying around from repaired signs which usually are in working condition.
In both cases, it is a good idea to go to the store in person and explain what you need the transformer for, as the people tend to be more helpful.
Instructions
Take both sections of the wire and straighten them out.
The method of connecting and stabilizing each section of the "V" may vary depending on the type of transformer you get, but the idea is the same.
Connect each wire to a high voltage terminal. Form a "V" that has a space about 1/4" at the bottom and 3" at the top.
The wires should not touch or the Jacob's Ladder will not work.
Use wood, glass, or plastic to stabilize the wires. Be careful when using wood, as the high voltage tends to carbonize it after a while, and when this happens, it may end up bursting into flame (as did the spark gap on my Tesla Coil).
A case can be handy because it keeps people from getting electrocuted and prevents the spark from extinguishing prematurely due to air currents.
If you decide to make a cover for the Jacob's ladder, be sure that it is an insulator, the spark can't touch any part of it, and that there are small vent holes at the top and bottom.
Here's a picture of my Jacob's Ladder with the shutter speed set at 1 second.
The wires come off the high voltage terminals of the 15kV, 30ma NST, are held in place by the top piece of wood, and form a "V".
If you're still confused, go to these other sites.
A SIMPLE JACOB'S LADDER from Larry Lund's Halloween Page-- Plans for Jacob's Ladder using an oil burner transformer.
Rob Burgess' High Voltage Site - My Jacob's Ladder is almost identical to the one on this site.
Jacob's Ladders - The Halloween-L Archives - Discusses Jacob's Ladders
Jacob's Ladder from Electrical Fun! - A Jacob's Ladder using a power distribution transformer. (overkill, but really neat. Kids, don't try this at home :-)
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