Date: Thu, 21 Oct 2010 14:32:23 +0100 From: Arthur Chance <freebsd@qeng-ho.org> To: RW <rwmaillists@googlemail.com> Cc: freebsd-questions@freebsd.org Subject: Re: Greybeards (Re: Netbooks & BSD) Message-ID: <4CC040E7.7090404@qeng-ho.org> In-Reply-To: <20101021133844.235fdc72@gumby.homeunix.com> 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>
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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. Air/brass would give very small differences in signal, and we're talking about the times when 7400 TTL logic with 4 gates per package was state of the art, so big signals were good. -- "Although the wombat is real and the dragon is not, few know what a wombat looks like, but everyone knows what a dragon looks like." -- Avram Davidson, _Adventures in Unhistory_
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