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Why dont Tesla coils arc to ground?, Colorado Springs

Hi Everyone,

Here's an interesting post I got off sci.e.ectronics about why Tesla arcs
dont jump straight to the ground but seem to go lengthwise instead. 


          Wednesday, 4 January 1995 11:58:56 AM
          sci.electronics Item
  From:           wesb-at-vnet.ibm-dot-com,News Only
  Subject:        Re: Nikola Tesla,  coils,  and other interesting analog
circuits/experinments
  To:             sci.electronics
In article <pferdD1uAzu.E1y-at-netcom-dot-com>, pferd-at-netcom-dot-com (Triggur) writes:
|> 
|> >John Lundgren (jlundgre-at-news.kn.PacBell.COM) wrote:
|> >: Brendan Haley (bhaley-at-shore-dot-net) wrote:
|> >: : I'm very interested in Nikola Tesla, specifically the tesla coil.  I
have
|> >: : a number of old schematics for the coil,  but they are extremely
outdated,
 
|> >: : and I am interested to hear from people who have built coils, or who
have
|> >: : more contemporary plans.
|> > 
|> >There's a company called Amazing Devices or something like that that 
|> >sells plans for these up to one that generates 2million volts and 15-20 
|> >foot sparks.  I'd say that's pretty impressive! :)  I think it stands 
|> > 5 to 6 feet high...
|> 
|> 
|> 
|> That's awfully hard to believe.  I built one 6 ft. high, ~1M volts, and it
|> threw sparks about 4 feet.  I tend to think that if it's going to throw
15-20 
foot
|> sparks, it'd MUCh sooner just arc directly to the ground or something.  :P

Wether or not the discharge goes to ground is dependent more on the
orientation of
the secondary than the length of the spark. A well designed Tesla Coil can
put
out a spark that's about 3 times the length of the secondary, provided that
the
secondary is vertical, and because of a couple helpful physical phenomena,
will
discharge upward or horizontally, but very rarely downward. The first
phenomenon 
occurs because an initial discharge into the air will
ionize it, providing a lower voltage conductive path which lets the discharge
go further, ionizing a longer streamer, providing a longer conductive path,
and
so on. 

The discharge is a lot longer than what you'd get with static voltage just
breaking down the air. In fact, it tends to be _roughly_ about 4 times the 
comparable length, so if the high efficiency Tesla Coils get too more
efficient, 
they'll experience discharge across the secondary or to ground, as you
suggested.
 
The second reason that Tesla Coils with such long discharges 
don't often have this happen is that the conductive ionized air that helps
make
such a long streamer is hot, and it wants to rise. Since the path across the
secondary (or to ground) requires the streamer to move downward, it doesn't
happen often, although a discharge into the air, rather than to another 
terminal can move pretty wildly. And since the distance across the secondary
is
too long for a direct voltage breakdown of the air across it (so far!), these

extra long streamers are quite possible. In fact there are a number of folks
in
the US who achieve such lengths routinely. (There's a group in Richmond, VA 
that probably get sparks that long before their coils are even in tune. I
think
they're crazy, but I love to see pictures of their stuff in action...)

I haven't seen the plans referenced by the original poster, but the numbers
seem
perfectly reasonable. What wasn't said was that a coil that can produce 15-20
foot sparks can't just be plugged into a standard wall outlet and it requires

elaborate ballasting to allow the coil to be brought up at sucessively higher
power levels as it's gradually brought into tune. This translates into a
control
panel that's far more expensive than the coil, a special power service for
the
coil (a discharge of that length would require 10-12 KVA if done
efficiently),
and a power line transformer operated in reverse to drive the coil. These
things
are absolutely unforgiving if you make a mistake, and your demise need not be
so predictable as an electrocution; you can also go in an explosion or a
shower 
of molten metal.

For some reason, I prefer a smaller coil, but to each his own...

In any case, these really big systems aren't for beginners, but they're quite
real and immensely impressive.

Wes B.



-- Mark
       _/_/_/   _/_/_/_/       Mark Conway
      _/    _/    _/          Deep Thought BBS, Auckland, New Zealand
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