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Re: High Voltage (fwd)



On Wed, 24 Sep 1997, William Beaty wrote:

It does take energy flow to maintain an arc, and the longer the arc, the
more energy per second. I've seen estimates for long Tesla Coil
discharges ranging between 300w/foot and 1Kw/foot. I don't know if this
applies to 18" sparks, I think it's more valid for sparks many feet in
length.

To clarify: tesla coil sparks are a special case, because they usually are
single ended, extending from a single electrode, and they form a dendrite
with numerous branches and tips. While extending itself, the spark acts
like a conductive capacitor plate, with the ground acting as the second
plate. A small irregularity on the main electrode acts to initiate the
discharge, and so a perfectly smooth polished torus electrode will need a
much higher voltage before sparks ignite. If a grounded electrode is
provided at a distance, it distorts the e-field and allows the spark to
reach to longer distances (or the ground electrode may send out its own
sparks to meet the main discharge.) Not a simple situation.

The spark stays in existence because the current is high enough to keep
the conductive plasma "alive". If I understand correctly, the peak
electron velocity inside the spark must be high enough to trigger electron
avalanches, and this is proportional to volts/cm within the spark. But
the voltage drop along each bit of the spark is not related to the tesla
coil output voltage in a simple way, while it *is* somewhat proportional
to current within the spark, and therefor to total watts. When it first
appears, the spark lengthens itself and the current within it falls, and
diameter rises, so the spark stops lengthening presumably when the current
nears the threshold where the plasma quenches. This implies that the
spark length is determined in part by output impedance of the coil, but
is also something like homeostasis in a biological system. If we feed it,
it grows, but the relation between the growth and the feeding is complex
and nonlinear.

From the appearance of long tesla coil sparks, I suspect that a
feet-per-watt constant exists, but that it only applies to the fully-lit
part of the arc, and not to the narrowing tips in the dendrite portion or
to arcs below about one meter. Of course dust, air pressure, and humidity
probably change this constant significantly.

Actually, when it comes to bragging about tesla coils, most hobbyists
usually abandon voltage output rating. It isn't that important (except
for impressing a nontechnical audience.) A wattage rating or a spark
length rating tells much more about the capabilities of a particular coil.

As an on and off TC hobbyist, I look at the psychology of building
enormous dangerous devices, and at the gender of most of those involved in
such things, and conclude that it's very appropriate to rate coils not in
terms of volts, but in terms of (ahem) ...length.

;)

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William Beaty voice:206-781-3320 bbs:206-789-0775 cserv:71241,3623
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