Re: cap firing voltage scope measurements question (tests)

"So if the safety gap can be
trusted to fire at the same voltage whether or not the TC is operating,
this then suggests that the scope readings are correct, and that the
cap voltage is actually higher when the TC is operating.  This is
perhaps not a definitive test, but it a clue nevertheless.  I am not
aware of any cap charging theory that explains this extra voltage
rise I'm seeing when the coil is operating."

	Very simple explanation of this very real effect.  With so-called
"matched capacitors" connected in parallel with the transformer, whose
reactance is approximately equal and opposite to the leakage inductance
of the transformer, you have a fairly high-Q 60 Hz resonant circuit, and
the unloaded voltage can rise to many times the the normal open-circuit
value.  Of course, at some voltage the transformer insulation will fail.
First order approximation to the leakage reactance of the transformer is
just (open circuit voltage / rated current, or about 250,000 ohms for a
15 kV, 60 ma transformer).

	With the safety gap across the transformer, the highest voltage it will
see is the breakdown voltage of the gap.  (Ignoring possible extremely
high frequency transients, which I suspect don't exist in practice.)  In
your case you have set the gap spacing to a "safe" value.