The main reason that skinny coils are difficult to get to
work is the very low coupling they provide.
Any coil, where the primary resonates exactly at the same
frequency of the secondary resonance, will eventually transfer
all primary energy to the secondary. If the frequencies are different,
not all primary energy will be transferred.
Low coupling coils are very sensitive to these frequency offsets, so they
have to be precisely tuned. This problem is aggravated by the slow
transfer of energy to the secondary by weakly coupled coils.
The arc will have time to break out and detune the coil by its capacitance.
So later during the burst it becomes off tune. By tuning you have the choice
of the coil either being in tune initially or later in the burst. A compromise is
hard to find for low coupling.
Bill Wysocks coil, Kurt wrote about, is of the magnifier type. There
is no magnetic coupling to the skinny secondary. What I've written above
does not apply in that case. Skinny coils can perform well then.
Udo
David Thomson wrote:
...Not having done any calculations on the eBay coil, my experience in working
with the geometries of coils reminds me of my own failures. I, too, burned
out a couple transformers on tall solenoid coils. Tesla ultimately isolated
the power circuit to the primary by making the spark gap totally
independent of the transformer frequency, such that he could generate
pulses at whatever rate he chose. I don't know how he did it, but I recall
him discussing this either in his Inventions book, or the Colorado Springs
Notes. The key to his eventual success was the shortest pulse at highest
energy and the right rate...