## [stella] Jacobs ladder (off topic)

 Subject: [stella] Jacobs ladder (off topic) From: Chris Wilkson Date: Mon, 3 Dec 2001 13:01:22 -0500 (EST)
```On Mon, 3 Dec 2001, John Saeger wrote:

> transformer from an old Microwave oven would work for something like this?
> I think they *only* put out around a KV or so, but they've got lots of
> current.

Hi John,

If you want to build a Jacobs Ladder, you want a high voltage.  The current
doesn't really matter (see below).  The power supply voltage has to be able
to overcome the dielectric (air) resistance to form a spark/arc.  Once the
arc is formed, it's essentially a zero resistance path.  The arc rises due
to the heated air/plasma rising.  So if you want it to rise faster, dump
more power into the arc.  In this case, the distance between the electrodes
sets the minimum voltage required, so the current will set the power.  Also,
most jacobs ladders are only a few feet long.  And the gap increases as you
go up.  So eventually there isn't enough voltage to maintain the arc.  It
breaks and starts over at the bottom - the narrowest point in the spark
channel.  If you make the 2 rods parallel (with balls or whatever at the
bottom to start the arc), the arc will still rise to the end of the rods,
no matter how long they are, but it won't break right at the end.  Instead
you'll get a really cool, scary, ribbony arc whose ends are still tied to
the rods, but whose center rises due to the heated air currents.  An arc
riding on an eddy is really awe inspiring!

The one we have at work uses an old oil burner transformer.  I know it
puts out 30mA, quasi-regulated, but I forget the voltage.  It's either 3kV
or 30 kV.  3kV seems right....but that might be under load....

Hmm...speaking of load, that's another contributing factor.  As the arc gets
shorter (with constant current) the voltage goes down...V=IR.  So a larger
current capacity actually will help you get longer arcs.  That isn't very
clear, but I have to go...sorry.  :)

-Chris

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