RE: Static gap behavior

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

Original poster: "Lau, Gary" <gary.lau-at-hp-dot-com> 

Hi Chris:

A 50 mil gap is not too small for a 4kV power supply, that is what I use 
with mine.  The arcing along the length of the gap is consistent with what 
I see on mine.  I've always thought it was strange that the gap arc 
wouldn't come to favor some point like one of the ends, where there is an 
edge, the temperature would be the highest and once started, the 
concentration of ionized gasses would be highest.  It's not clear what 
"motivates" the arc to move from any one place to another.  I'm just happy 
that it does!

There are tables that predict breakdown voltages between various electrode 
shapes (see http://hot-streamer-dot-com/TeslaCoils/Misc/SGapVolt.jpg), but 
there are so many variables that affect this, that its better to just hook 
up your NST to the gap and adjust it with no tank circuit until it just 
fires.

Have you tried blowing air at the gap with a small fan?  Beyond using the 
correct cap size and getting the tuning and coupling right, I've found that 
top load size matters a lot!

Gary Lau
MA, USA


Original poster: "C.T." <ct451-at-yahoo-dot-com>

I'm using two horizontal 4.5" pieces of 1" copper pipe as a gap on a small
coil running on a 4000/30 NST.
The gap is too small <0.05" apart and I only get 6-7" streamers on the top
but I noticed a peculiar behavior while the coil is running.
The actual spark  will only be ~3/4-1" along the length of the pipe but it
starts on one end and slowly moves up length of the pipes over a number of
cycles (or so it appears to the naked eye).
When it reaches the end it will either turn back and come down the opposite
direction or jump to the other end and start moving forward again.

More ionized gasses at the side of the previous spark or something I'd guess.
Is this normal behavior for this type of gaps or has anyone observed the
same? How does this affect the performance of the coil?

Also I was wondering if there's a rule of thumb or a table somewhere that
gives approximate breakdown voltages for gaps with cylindrical electrodes.

any input welcome,
Chris