On Aug 28, 6:25*am, Tim Shoppa wrote:
On Aug 28, 7:29*am, raypsi wrote:

On Aug 27, 3:58 pm, K7ITM wrote:
I'd bet that the shorted-turn effect is very much larger than the
permeability effect with respect to changing the inductance of the
oscillator coil.

Cheers,
Tom

I agree, except Air has no permeability, you can't magnetize air. You
can't magnetize brass.

I think you're confusing permeability with susceptibility. Even vacuum
has non-zero permeability.

I'm still considering Tom's points. What I know from decades of
experience, is if I put a brass slug into a solenoid, the inductance
goes down by a little bit. "Little" is relative to the effect of a
ferrite slug, which makes inductance go up by a lot in comparison.

The "little" is key for the PTO designs I mention, it allows rather
fine tuning with very simple mechanicals. If I think of it as shorting
turns as it moves in, that is completely out of whack in my head with
the observed changes, because shorting all the turns causes a big
change in inductance, not a small one.

Tim N3QE

Hi Tim,

OK, a 'speriment for you, and another way of looking at it that will
yield the same result:

Make two "slugs," one solid and one hollow. If it's permeability
that's doing it, the hollow one (with thin shell, of course) will have
considerably less effect. If it's the "shorted turn" effect, both
will be about the same.

Consider that there is no time-varying magnetic field inside a shell
made out of good conductor--really zero if it's a superconductor, but
dropping to practically zero after a few skin depths. So you are
removing a volume of magnetic field when you put a conducting slug in
the coil's field. With less field, there's less energy stored, which
implies lower inductance. The same thing happens when you put a coil
inside a shield-can: the inductance is reduced. A points that seems
to be not well known: the unloaded Q of a helical resonator is less
than the Q of the same coil that's not inside a shield. There are
graphs available to determine the lowering of inductance by a
cylindrical shield around a coil.

Cheers,
Tom