Re: [stella] Jacobs ladder (off topic)

Subject: Re: [stella] Jacobs ladder (off topic)
From: Mark De Smet <de-smet@xxxxxxxxxxxxxxx>
Date: Mon, 3 Dec 2001 19:55:18 -0600 (CST)
Sorry for continueing an off topic thread, but there is not much traffic

> > BTW, I had a question about your Jacobs Ladder.  ;-)  Do you think a
> > 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.

I am unfamiliar with the specifics of such a transformer.  I did a bunch
of research before building mine to verify a lot of facts.  Important
things to consider when building one:

-Transformer must be current limited!  As Chris points out, once the arc
starts, there is basically a short on the output of the transformer.  This
will blow a normal transformer.  There are 2 common uses for current
limited transformers, neon signs and arc welders.  Both types limit the
current by having a secondary flux path for the magnetic fields.  This
makes it reluctance controlled based on the output coils and the mechanics
of the core.  Short of the engineering required to make a current limited
transformer, I would recommend just finding one of the above types.  Neon
sign transformers work best

-I don't see any reason why you couldn't limit the current with resistors,
but that will make some pretty high wattage resistors, and the added
connections make it a little more dangerous to deal with

-Ozone!!!  Most people don't know about this.  An electric arc is a plasma
which is a slurry of atoms where the electrons are no longer orbiting the
nucleous'.  When these atoms revert back to the gaseous state, there are
no gaurntees about what molecules they form.  Oxygen is the biggest
player.  Most atoms revert to O2, but some recombine as 03(ozone).  Ozone
is a carcinagin.  Thankfully ozone is very pugnent, so you will smell it
long before it poses a health hazard, but it must be considered.  You can
smell ozone by running a old power drill at high speed, and smelling the
cooling exaust.  Indoors w/ the windows closed, I don't leave mine on for
more than a couple minutes.

-As Chris points out, the voltage will change the bottom gap.  My 17000V
can just cross the 1/2 inch at the bottom.(I don't remember the equations,
and don't feel like looking them up.)  

-Current will affect how fat the arc will grow, and how large the top gap
can be.

-Protection.  No, you don't want to get shocked with this much voltage!
(as a side note, a high DC voltage is more likely to throw you against the
wall and kill you by breaking your neck/other trauma; and a high AC
voltage is more likely to stop your heart.  But this is a whole different
topic.  I don't recommend trying either)

> 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

It breaks because the plasma disperses.  A higher current will better
maintain the plasma(and in a fatter arc) to allow it to go higher.


> 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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