Secondary Elevation

Hello TC experts,

To get the maximum performance from a TC and to eliminate damaging
secondary "racing sparks" along its surface, the pri-sec coupling needs to
be optimized.  This is typically done by varing the height of the bottom of
the secondary winding in relation to the plane of the primary winding
(assuming a flat spiral).  Some questions:

1) Is there a rule of thumb or empirically derived equation which gives a
good starting point for pri-sec height difference for a given sized
secondary?  E.g., something like height difference = sec diameter/4?

2) I would appreciate ideas you have incorporated into your coils for
varying the pri-sec height differences.  I am interested in building a
research coil which has this parameter adjustable in, say, quarter inch
increments.  Ideas for either adjusting the secondary or primary height,
while still retaining physical stability and esthetics of both, are what I

Thanks in advance,

--Steve Young

P.s.  I suppose the best answers to these questions would be candidates for
the FAQ compilation.