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Date:      Thu, 21 Oct 2010 15:57:24 +0100
From:      RW <rwmaillists@googlemail.com>
To:        freebsd-questions@freebsd.org
Subject:   Re: Greybeards (Re: Netbooks & BSD)
Message-ID:  <20101021155724.0eb1c5a1@gumby.homeunix.com>
In-Reply-To: <4CC040E7.7090404@qeng-ho.org>
References:  <op.vj5o9ixxhtl4zj@ack5833s2.ad.service.osu.edu> <20101017143901.GA71132@current.Sisis.de> <alpine.BSF.2.00.1010171215030.96626@wonkity.com> <20101019074615.GA2183@current.Sisis.de> <alpine.BSF.2.00.1010191448390.6689@wonkity.com> <20101020022946.GA23035@thought.org> <20101020052601.GA1977@current.Sisis.de> <4cbe9e9a.3qT7q8JUqJxSD8/V%perryh@pluto.rain.com> <20101020165526.GA25310@thought.org> <4CBF21EB.1080003@tundraware.com> <20101020194605.GA78565@stainmore> <4CBF4CB4.6070902@qeng-ho.org> <20101021133844.235fdc72@gumby.homeunix.com> <4CC040E7.7090404@qeng-ho.org>

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On Thu, 21 Oct 2010 14:32:23 +0100
Arthur Chance <freebsd@qeng-ho.org> wrote:

> On 10/21/10 13:38, RW wrote:
> > On Wed, 20 Oct 2010 21:10:28 +0100
> > Arthur Chance<freebsd@qeng-ho.org>  wrote:
> >
> >
> >> 50s) had the experience of programming microcode on a machine by
> >> inserting brass slugs for 0s and ferrite slugs for 1s on a pin
> >> board.
> >
> > I wonder why it was brass/ferrite rather than brass/empty or
> > ferrite/empty.
> 
> 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. If it is,
> then it would have the opposite effect and reduce the inductance, so
> you'd get a better difference in signal between brass/ferrite than
> air/ferrite. 

Possibly. I'm wondering if there might be three states, where the third
state is writable.

> Air/brass would give very small differences in signal,

I was thinking in that case it would be open/short circuit.



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