Static gap behavior

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

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