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Sparks in air The dielectric breakdown strength of dry air, at Standard Temperature and Pressure (STP), between spherical electrodes is approximately 33 kV/cm. This is only as a rough guide since the actual breakdown voltage is highly dependent upon the electrode shape and size.
 A. H. Howatson, "An Introduction to Gas Discharges", Pergamom Press, Oxford, 1965, no ISBN - page 67
That value is approximately what I first referenced, 3 kV/mm. Normally, I'd try to quote something from NIST, but I forgot where to find it in their pubs. The other source that came up in my Google search had same value. So, that would convert to ~75 kV per inch, definitely different than your value. I'd bet your value relates to a specific situation or is intended as a safety rule-of-thumb (hence more conservative). Do you know where you get 25 kV/inch? For the record, main reason I answered was to cover the safety issues! Such a simple device is actually a complex safety concern, particularly the UV emission and Ozone creation. Had a friend in undergrad that sunburnt her eyes working in a lab. She was viewing slides on some sort of light-table that was outputting unsafe UV. She had to take a medical withdraw for the entire semester!
Once an arc is initiated, only about 20 V/cm or 50 V/in is required to maintain the arc regardless of the magnitude of the arc current. 
 J.D. Cobine, Gaseous Conductors. New York: Dover 1958.
So only 50 V per inch! is necessary to keep the arc, leading to a second estimate of 500 V for a 10 inch spark, even less than my first approximation of 5000 V. Now I'm starting to believe the numbers... SO... for the initial question, it looks like no exotic transformers are necessary to achieve a 10 inch spark. Enough to overcome the initial spark formation is the major voltage hurdle. ADVICE:
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Posted:Oct 25, 2010
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