Voltagee measurement

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Kamil Kompa by way of Terry Fritz <twftesla-at-qwest-dot-net>" <tcmail-at-poczta.fm>

Hi !

I have a trouble with a transformer. I don't know it's parameters, so I
want to measure voltage 
of the transformer (i think 5 to 20 kV). How can I do this?
I found that spark apears between the electrodes connected to the
transformer when the gap is about 15mm (between sharp nails) and about 6mm
gap is required when electrotes are two spheres (no sharp edges).
I know that breakdown in air is about 30V per 0.025mm, but which distance
schould I use for accountings (6mm or 15mm)? 
I also noticed that when nails aore little wider than 15mm on the top of
one of them appears corona. (sparks striking to the air in the direction of
the other nail). There is no corona when I use spheres separated with more
than 6mm of air.
Could you tell me something about the output voltage of my transformer
having this data?
Thanks for all answers.

Kamil Kompa