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Re: spark length vs toroid diameter

Subject:   Re: spark length vs toroid diameter
  Date:   Thu, 8 May 1997 15:33:56 -0400
  From:  "Thomas McGahee" <tom_mcgahee-at-sigmais-dot-com>
    To:  "Tesla List" <tesla-at-pupman-dot-com>



----------
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-poodle.pupman-dot-com
> Subject: Re: spark length vs toroid diameter
> Date: Thursday, May 08, 1997 2:17 AM
> 
> Subject:     Re: spark length vs toroid diameter
>       Date:  Wed, 07 May 1997 18:58:24 -0800
>       From:  Greg Leyh <lod-at-pacbell-dot-net>
>         To:  Tesla List <tesla-at-pupman-dot-com>
> References: 
>            1
> 
> 
> dw pierson wrote:
> 
> >         Assuming the rest of a given coil setup can provide some
> >         arbitrarily high voltage (eg: low enuf losses, high enuf
> >         power), the spark over distance is set, largely, by the
> >         terminal (toroid, sphere, whatever) diameter.  A 'zero
radius'
> >         terminal (point) will spark out at a relatively low
voltage.
> >         If LOTS of power is available, there may be long sparks. 
A
> >         larger toroid will 'allow' higher voltages before
sparking over.
> >         (Of course changing toroid size also changes tune, all
the
> >         way back thru the system, so simply dropping on a new
toroid
> >         to see the effect can be complicated to interpret...)
> 
> 
> There are several points to consider here:
> 
> 1)  Why then wouldn't a hemisphere on the top of the coil be the
best?
> 

A FULL Sphere would be best as far as CAPACITANCE goes. It would also
have the edge as far as BREAKOUT of the discharges goes. But a sphere
is lousy at shielding the top windings of a Tesla coil, compared to a
toroid. The sphere has a greater radius of curvature for a given
diameter size, but it also takes up more room, uses more metal (or
whatever you make your top loads out of...), and provides less
shielding (for a given diameter size, of course).

By "hemisphere" you might mean the configuration where the top half
is perfectly hemispherical, and the bottom "half" is more like the
BOTTOM of a toroid. This would have more useful capacitance than a
toroid of same diameter, but less than a full sphere. It would have
shielding equal to the toroid, which would be an improvement over the
full sphere. They use this shape a lot with commercial Van de Graaf
generators, as it reduces the tendency for discharges to shoot down
the outside of the support column. Note that such a shape is not as
easy to build out of the kind of glorified junk that we coilers tend
to use as our materials of choice.

> 2)  Some coilers report getting longer arcs by putting breakout
points
>     on their toroids, which actually compromise the hold-off
voltage.
> 

The breakout points focus the breakouts to a smaller area of the
toroid. That will cause the ionization near that area to be somewhat
increased. Ideally the breakout point should be a BUMP rather than a
point. A point will start corona discharge early and drain off energy
that COULD have gone into actual discharge production. Note that
sometimes a point's ionization may actually scare up a discharge that
is longer than normal... depends on many conditions. But in general a
bump is better than an actual point.

> 3)  The streamer itself, once formed, acts as a sharp wire hanging
off
>     the toroid, further compromising its holdoff voltage.
>

Yes, the discharge compromises holdoff voltage, but the same
phenomenon also allows the discharge to GROW on itself, so to speak,
over multiple oscillations.
 
> It would seem that a larger radius of curvature isn't necessarily
> the reason for longer arcs.  What else could it be?
> 
> -GL

A big old humongous sphere is still one of the absolute BEST toploads
you can put on a Tesla coil. But toroids have so many things going
for them, that they are the topload of choice for coilers. I believe
that the shielding action of the toroid is one of its best features.
You want max capacitance, go with a sphere. You want close to max
capacitance with less size and better shielding, then go with the
toroid.

I LOVE to steal the spheres off of Van De Graaf generators and use
them as toploads. But the physics department always comes along and
demands that I give them back! Maybe if I built them a Van de Graaf
generator using a toroid they would be happy? Nope, it has to look
like the thing in the textbook or they get totally confused. Poor
souls!

Fr. Tom McGahee
(I don't actually "steal" the spheres... I just borrow with
permission and keep for as long as I can)