How to measure transformer power..

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

Original poster: "Simonas by way of Terry Fritz <twftesla-at-qwest-dot-net>" <simpastas-at-takas.lt>

hiall!
 
I'm new coiler, recently I built my second coil and there are some questions
somebody maybe could help me with..
The label on main transformer says "100/10000, 640VA", it's old (made in 1958),
heavy (36kg), Russian made transformer, I have no idea where it was used
before, but I hope it's capable of giving more VA than it's rated..  Since I
have no variac, I connected it in series with powerful 14 Ohm variable
resistor. Without load on the secondary  this setup gives 150 V on the
transformer (and 15kV output) and 75 V on the resistor, input current is about
7A.. With load (for example multi static gap adjusted to fire constantly
connected directly to the secondary winding) current in the circuit raises to
16A and voltage on the transformer drops to ~30V.. I'm totally confused, I
can't measure transformer output voltage and current directly but I would like
to know them in order to tune my coil..  
Coil is 4.5X18 inches secondary (900 turns), 0.01175 uF tank capacitor (four
0.047uF 16kV connected in series) and flat 10 turns primary tapped on the 8.5
turn (6'' inner diameter and 17'' outer), toroid is rather small (6pF). Coil
produces 30'' sparks (adjusting variable resistor during operation to 7 Ohm) ,
but as I understand it's capable of much more.. therefore I need to know power
I'm feeding into that coil (not the resistor :) because it gets bright red
during operation..)..
 
thanx in advance.. 
 
P.S. when I connected whole primary circuit together (with static gap adjusted
to fire constantly when connected directly to xmfr) there was no firing at all
(??) I had to narrow spacing in static gap almost by half (??).. if I
understood right, conecting xmfr with capacitor should give something like
resonant rise and increase voltage in the primary circuit, but in this case
it's all vice versa..