Off-axis tuning coil? (was: I Think I Messed up!)

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

Original poster: "Patrick Leonard by way of Terry Fritz <twftesla-at-qwest-dot-net>" <transactoid-at-rogers-dot-com>

Someone suggested I use an additional coil in series with my primary to
increase the inductance. I've only ever seen one or two other coil like this.
It had maybe only 4 or 5 turns, and used an additional coil of maybe 7 or 8
turns from which a wire was used to tap the secondary. I think the builder
called it an off-axis "tuning coil".  One of the coil's on
<http://www.hvguy-dot-com>www.hvguy-dot-com also has such a "tuning coil".
 
How would I calculate the dimensions, or is this something I can just kind of
guess at? Also, is it wound flat spiral like the primary or cylindrically?
(that's how the above tuning coil was made).
 
Lastly, how does such an addition work? >From what I understand, the flat turns
of the primary generate a magnetic field which induces a current flow in the
secondary. I don't see how a seperate coil, located somewhere else, not in the
"plane" of the secondary transfers any energy over?? Am I mis-understanding
inductance?
 
Thanks,
Patrick.