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From: will@nscpdc.UUCP (William Hobbs)
Newsgroups: net.followup
Subject: Re: Jacob's Ladders
Message-ID: <604@nscpdc.UUCP>
Date: Fri, 19-Sep-86 12:02:03 EDT
Article-I.D.: nscpdc.604
Posted: Fri Sep 19 12:02:03 1986
Date-Received: Sat, 20-Sep-86 04:40:50 EDT
References: <1249@utastro.UUCP>
Reply-To: will@nscpdc.UUCP (William Hobbs)
Distribution: na
Organization: NSC Portland Development Center, Portland Oregon
Lines: 45
Keywords: how to ...

I built a Jacob's ladder a couple of years ago. The source of voltage was
a neon sign transformer rated at 15,000 Volts at 30 milliamps.

The primary of the transformer was connected to the AC line, with a
power switch and a 2.5 amp fuse.

The secondary has ceramic standoffs that insulate it from the metal
casing of the transformer. The contacts of the secondary are small bolts,
which I attached to a couple of 1/4" steel rods. The rods were then bent
around the transformer (the ceramic standoffs are at opposite ends of the
transformer), keeping them at least two inches away from the transformer
casing. The rods were bent until they reached the middle of the top of the
transformer, and then given a shrp upward bend into the characteristic
"V" shape. At their closest point, the rods are about 3/8" apart, and
they diverge to about 2" at the top of the V. 

To hold everything in place, I took an old speaker cabinet (approx.
18" X 10" X 6", placed it on its side, cut a hole in the box above
where the middle of the transformer would be, and mounted the transformer
inside. Covering the hole (which measures about 3" X 3") I put a nylon
block that had small holes drilled in it for the steel rods. The steel
rods then pass through the nylon block, bend toward their closest point
at the bottom of the "V" and go up about 18" from there. I enclosed the
rods in a glass tube that is slightly longer than the rods.

The arc produced by the Jacob's Ladder is quite strong, and could easily 
cover a much wider gap at the top, but I was limited by the glass tube
I had.

When you turn on the Jacob's Ladder, the voltage is high enough to bridge
the short gap at the bottom of the two rods. Current passing through the
air at the gap ionizes the air, making it a low resistance path for the
current to flow through. It also heats the air, causing it to rise. Since
the ionized air conducts the current better than un-ionized air, the arc
follows the air up the ladder until the gap is too great for the arc to 
bridge. At this point, the arc breaks, and starts again at the bottom.
Quite a conversation piece.

Incidentally, I got the transformer at a garage sale for $5.00, but
any neon sign company should be able to steer you to one.

				Good Luck, Will

    Will Hobbs, KE7OC 			nsc!nscpdc!will
    National Semiconductor		tektronix!reed!nscpdc!will