The at 1 mm of Hg the breakdown voltage is quite a bit less that at atmospheric pressure 760mm. You could not tell the difference for a perfect vacuum and 1 mm by pulling on the shaft. 73 Bill wa4lav -- FAQ on WWW: http://www.contesting.com/FAQ/amps Submissions: amps@contesting.com Administrative requests: amps-REQUEST@contesting.com Problems: owner-amps@contesting.com