I'm certain the voltage did not double. If that were the case the
transformer would have let out it's magical blue smoke and ceased to
function. It seems to me that most of the extra voltage is going directly
into current. When I used the transformer to drive a Jacobs ladder the
result of pushing the voltage all the way up was that the arcs became much
hotter, and I was able to measure a small amount of current with my clamp
meter. When I was running the Jacobs ladder I did not let the current on
the 120V side of the variac go over 20A I it usually reached 20A around
210V. I run my transformers in a sealed enclosure, and I opened this up
after overdriving them for quite some time and did not note the presence of
ozone. Aside from the transformers being slightly warm the overdriving
didn't seem to affect them that much. I probably ran the transformers at
200V+ for at least 30 minutes or so. I'll try taking voltage measurements,
but with my primitive setup I can't promise any results.