On Thu, 19 Apr 2007 19:31:45 -0500, "amdx" wrote:

snip

I have no compelling reason to build a variable inductor. It's just a
curiosity.
If the potcore/toroid idea was feasible, I might try using it as a magnetic
mixer
for a radio front end.
Maybe a better question would be; What's the best geometry to bring the
core
of one winding into saturation with the flux from seperate winding.

Google 'saturable reactor' or 'magnetic amplifier'.

Your proposed construction isn't out of the question for a
small-signal circuit.

The most common form is that used in conventional television
where two out of phase windings on the outer legs of an E-core are
used to control the permeability of the signal winding present on the
center leg.

The next most common is the two toroid structure, where control
windings are wound on two separate toroids; these are sandwiched and
used as a single toroid structure for the main signal or power
winding.

RL