On Thu, 28 Aug 2008, Tim Shoppa wrote:

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.

Which of course is why you see iron core and ferrite slugs in high
inductance coils, but in small inductance, like you'd see at VHF,
the slugs were brass.

Or, that tuning wand with some sort of ferrite at one end and brass
at the other, so you could tell whether the tuned circuit was too high
or too low.

At that level, brass and ferrite were seen as the same thing, a means
of varying the inductance. The choice was made by how much variation
you needed, and which way, rather than what action was taking place.

Michael VE2BVW