calculating safe primary turn-to-turn distance

["Stan" <sdarling-at-columbus.rr-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Greetings all,

I have been curious since I started doing Tesla coils about this:  how
does one calculate the minimum air distance of turn to turn spacing in a
primary for a given voltage?  I'm not an EE so my common sense tells me
that current always favors the path of least resistance and that even a
tiny distance of air would have a much greater R than the adjacent half
turn of Cu tubing. So if the adjacent turns weren't touching, why would
it ever arc over through the air?  I suspect it has something to do with
the inductance of the Cu tubing coil(s) ....

Feeling stupid in Ohio.

-Stan