Re: Gap Arc Voltage Was: Natural streamer

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz <twftesla-at-qwest-dot-net>" <acmq-at-compuland-dot-com.br>

Tesla list wrote:
> 
> Original poster: "Barton B. Anderson by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <tesla123-at-pacbell-dot-net>
> 
> Hi Carlos, All,
> 
> Sorry to post this so late on this subject (of sorts), but I've been
> thinking about coilers spark gaps and arc
> voltages. Typically, a coiler will use either electrodes with a large
> radius (of which the ball-ball equation fits
> nicely as per my own experimentation and works for ball-ball safety gaps),
> but there are coilers which use electrodes
> with a flat edge as well. I'd like to think a plane-plane gap equation
> would suffice, but considering the electrodes
> are typically 1/2" down to 1/8", I'm not sure a plane-plane arc voltage
> equation would work well (then again, to a
> spark, that small flat might appear pretty large). I guess I'm curious for
> thoughts on the subject from you or anyone.

A plane-plane gap would spark at 30 kV/cm, or a bit less, from DC to
at least 300 MHz, so the frequency of a Tesla coil doesn't make
difference (for single sparks).
Power, heating the electrodes and the air between then, makes a big
difference. Breakdown voltages are inversely proportional to the
absolute temperature (Kelvin) and directly proportional to the
air pressure. Consider that the temperature of a primary circuit
gap in a Tesla coil can reach two or three times the ambient 
absolute temperature, maybe more, and the breakdown voltage would
get divided by the same factor.
But a plane-plane gap must be short relative to the widths of the
terminals, and the edges of the electrodes shall be rounded adequately
to avoid electric field concentration there (there is an optimal profile
for this). With sharp corners, the gap may operate as a needle gap, that
starts to emit corona as soon as the voltage exceeds ~300 V, and
eventually start to operate as a ball gap, with the "balls" being 
formed by ionized air around the needles. A rule of 10 kV/cm can
be applied to needle gaps in the usual conditions.

Antonio Carlos M. de Queiroz