Original poster: "Richard Modistach" <hambone-at-dodo-dot-com.au> the normal convention is a rating of 30,000v per cm to jump a gap. i built a negative vacuum type gap for the 4" coil i'm building and hooked my 15\30 nst up and runs quite happily at 15.5mm under air blast. does this mean i'll have to run it at 16mm or more in service?. richard