In response to Steve's elog entry, and for 40m posterity, I provide the Paschen Curve.
% Paschen Curve plotting % From http://home.earthlink.net/~jimlux/hv/paschen.htm % Breakdown voltage: % Vbreakdown = B * p * d / (C + ln( p * d)) % % Breakdown field strength: % Ebreakdown = p * ( B / ( C + ln ( p * d))) %