Update: I increased the ballast core mass, and got a little better
spark output, but the spark still only reaches 39" instead of 42" as
it did with the one potential transformer. It should be better
with the two transformers I'm using now, not worse. I might still
need a little more core mass in the ballast. I'm probably getting
more transformer core loss too now, since there are two cores
because I'm using two transformers.
The tests below were performed using the sync rotary gap at
120 BPS. Input power was 640 watts maximum as measured
with a lab-type wattmeter.
Anyhow, I went ahead and re-installed the new 4" by 13" spun
aluminum toroid, and now the spark does break out, just before I
reach the full variac setting. Without a director tab on the toroid,
the sparks tend to be attracted towards a wall behind the toroid,
opposite the measuring wire, but the sparks do emit in all directions,
and there are a few multiple simultaneous streamers, and good
spark action. Next I installed a spark director tab, then the toroid
produced only one streamer, and it reached the measuring wire at
Conclusions: The TC gives the same spark length with this 4" by
13" smooth spun toroid, as it gives with the 5" by 20" dryer duct
toroid. I know from the past that a 4" by 17" dryer duct toroid also
gives the same spark length. I do not have a 4" by 13" dryer duct
toroid to compare. Spark break out occurs much easier and earlier
using corregated dryer duct toroids, than using smooth spun toroids.
It is easier to obtain only one streamer with smooth toroids than it
is with corregated toroids. Streamers from smooth toroids drift and
move about the toroid surface whereas streamers from corregated
toroids tend to remain rooted in place. It is easier to fully direct the
spark from a smooth toroid using a director tab, than to direct even
a larger corregated toroid. Of course higher break-rates will tend to
form only one streamer with any toroid type (the minimum toroid size
to obtain just one streamer will be smaller).