From: sam@colossus (Sam Goldwasser)
Newsgroups: sci.electronics
Subject: Re: Jacob's Ladder
Date: 10 Aug 1995 13:19:47 GMT

In article <40b70l$lo4@ixnews3.ix.netcom.com> LDBarret@ix.netcom.com (Loren Barrett) writes: 

>   Can anyone provide plans to build the lightning generator called Jacob's 
>   Ladder? I want to make a small one for classroom experiments. Please 
>   e-mail to LDBarret@Netcom.com or fax to 702-294-3072. Thanks
You will need 12K-15K volts AC (50/60) Hz at say 20-30 mA. A neon sign (luminous tube) transformer is the usual source though an oil burner ignition transformer will work in a pinch (some say better and cheaper) or you could build an inverter type power supply.

Take a pair of thick wires - the steel wire from old metal coat hangers works pretty well - straighten them out and mount them with a gap of about 1/4 inch at the bottom and 1-3 inches at the top. Of course, all on an insulated non-flammable material! Connect the high tension output of the transformer to the two wires and you should be all set. Some adjustment of the spacing may be required (but do so with power off!). Depending on the voltage and power rating of your high voltage souce, these dimensions may vary considerably. 


                       __    1-3"
                        ^  \     /
                2-3 feet    \   /
                or more |    \ /
                        v  --| |--
                             1/4"
A jacob's ladder works on the principle that the ionized air in the arc is a lower resistance than the air around it and heated air rises. The arc strikes at the point of lowest breakdown voltage - the small gap at the bottom. The heated plasma rises and even when it is an inch or more in width is an easier path for the current to follow. Eventually, the gap becomes two wide, the arc extinguishes and is reestablished at the bottom. For best results, shield the whole thing from drafts but don't use anything that can catch fire!

Safety:

Make sure that no one can come in contact with this - particularly curious onlookers. Separating the potential victims from any possible contact with the high voltage is really the only foolproof way of protecting against fools or the unaware - and you from a lawsuit. People not familiar with high voltage phenomina (or aware only through C grade sci-fi movies) can be incredibly naive.

A GFI (ground fault interrupter) is of no use in protecting against HV contact since the secondary of a neon sign transformer is isolated from the line but its centertap is usually connected to the case - which should be grounded. However, a GFI would be a good idea in any case when you are working with line connected equipment.

12,000 volts will jump approximately 1/2-3/4 inch in dry air - more under humid conditions. Don't forget that 12,000 VAC is approximately 17,000 V peak. Neon sign transformers have current limited outputs - 30 mA is typical - but that is still highly dangerous - lethal under the wrong conditions.

You can build a small Jacob's Ladder using a high voltage transformer of lower capacity or a DC-AC inverter using a TV flyback transformer. While these would be less dangerous, there is little room for carelessness when working with high voltage. Even if there is no resistive path, the stray capacitance can permit enough AC current to flow to give you a painful experience!

--- sam