Path: utzoo!attcan!uunet!ginosko!ctrsol!cica!iuvax!mailrus!accuvax.nwu.edu!lunde
From: lunde@accuvax.nwu.edu (Albert Lunde)
Newsgroups: comp.sys.mac
Subject: Re: Mac as airline hand baggage
Summary: Magnetic field geometry and the risks
Message-ID: <975@accuvax.nwu.edu>
Date: 30 Jul 89 18:53:01 GMT
References: <10938@polya.Stanford.EDU>
Sender: news@accuvax.nwu.edu
Reply-To: lunde@nuacc.acns.nwu.edu (Albert Lunde)
Organization: Northwestern Univ. Evanston, Il.
Lines: 25

In article <10938@polya.Stanford.EDU> kaufman@Polya.Stanford.EDU (Marc T. Kaufman) writes:
>... Second the electric field set
>up by the detector coils is far too weak to erase a disk or tape.  There is
>no appreciable magnetic field, and anyway, magnetic fields attenuate as the
>FOURTH power of distance.
 ^^^^^^^^^^^^^^^^^^^^^^^^
I think geometry is important here - I think the magnetic field from a
finite dipole source varies with distance as the third or fourth power
depending what orientation you look at. But the situation here is different:
one is passing between large diameter loops, whose spacing is less than
or equal to their diameter.  Distant memories of physics experiments suggest
this is a cookbook setup to get nearly constant field strength over a large
volume.  Presumablely the field is varied at high frequency to maximize 
induced eddy currents.  This could also increase the likelyhood of erasure.

Northwestern University has one of our computer labs in a library, and one
of the standard things the consultants recovering trashed disks ask is if
someone has taken them thru the library security devices - there is
oral tradition to support the idea that this is a real problem, but I don't
know of anyone doing a controled experiment to test it out.  The library
and anti-shoplifting devices may differ in detail since they are looking
for a small item of known composition, not guns, knives , etc.

Albert Lunde                                         lunde@nuacc.acns.nwu.edu
"We are all part of one another" Barbara Demming