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Date: Fri, 22 Oct 2010 10:09:25 -0700
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From: David Brodbeck <gull@gull.us>
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Subject: Re: Greybeards (Re: Netbooks & BSD)
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On Thu, Oct 21, 2010 at 6:32 AM, Arthur Chance <freebsd@qeng-ho.org> wrote:
> Dredging up physics unused for 30+ years, ferrite is ferromagnetic and
> intensifies magnetic fields so a coil of wire with ferrite inside is a
> massively bigger inductor then an empty coil. I vaguely remember that brass
> is slightly diamagnetic, but could be mistaken.

Bingo.

A ferrite slug increases the inductance of a coil, relative to an air
core; a brass slug decreases it.

I've got a homebrew amateur radio transceiver that uses a coil with a
brass machine screw in it for tuning.  Wind the screw out of the coil,
inductance goes up.  A cheap and simple form of permeability-tuned
oscillator.