Original poster: "David Thomson" <mailto:dwt@xxxxxxxxxxxx><dwt@xxxxxxxxxxxx>
Hi Tim and Gary,
The two primaries could be mounted on Plexiglas rings and joined
at the center by a vertical rod, which the upper coil slides
against. To get maximum inductance variation, wind the coils in
opposite directions. Winding them in the same direction will
cause the inductance to become too weak when they are next to
each other. Winding flat spirals in opposite directions will
give inductances up to four times the inductance of a single flat
spiral coil. The connections would be to the outer windings of
both coils.
The rings can be raised and lowered via a rubber inner tube
filled partially with dry air. The upper Plexiglas ring can also
be made to slide up and down plastic rods with nylon string and
pulleys providing the remote lifting and lowering.
Dave T.
> I've not given any significant thought to the specific
> mechanism used to elevate one of the primary coils. I only
> meant to point out that such a mechanism which involves no
> moving electrical contacts would be easier to construct than
> one that continuously makes contact with a spiral coil. Yes -
> armchair coiling is easy ;-)
>
> >
> >> A far simpler approach would be to construct >> two
> conventional flat primary coils, joined in the center with a
> short >> length of flexible cable, and have a mechanism to
> vary the separation >> between them. This will vary the
> mutual inductance, and consequently, >> the net inductance
> of the two coils in series. The achievable >> adjustment
> range is probably equivalent to a couple of turns, and that
> >> should be sufficient for fine tuning. Rough tuning would
> be achieved by >> tapping the upper and lower coils in the
> traditional sense, with power >> off.
> >>