[Techtalk] Sending floppy disks thru the mail

[Conor Daly]

On Mon, Sep 16, 2002 at 11:58:24AM +0100 or so it is rumoured hereabouts, 
Maria Blackmore thought:
> On Sun, 15 Sep 2002, Conor Daly wrote:
> > Um, you know how we're talking here about floppy disks that use _magnetic_
> > storage and about shielding them by using steel which is itself a
> > _magnetic_ (ok, magnetisable) material, won't the steel just focus any
> > magnetic field and, especially if there are surface imperfections, maybe
> > increase the risk of damage?
> 
> The steel doesn't have to be magnetised :)
> 
> Anyway, the idea of using steel is that since it's a ferro magnetic
> material, it will effectively short circuit the field lines of any nearby
> B field.  This means that the field lines will not cut those of the tiny
> little magnetic domains in the coating on the disk, and not interfere with
> their so far peaceful lives :)
 
Ah!  Kinda like the magnetic version of a Faraday Cage...

> This is a very useful effect, it's used to produce completely magnetically
> shielded rooms for various studies, as well as the most important use,
> which is making transformers a lot more efficient that they would be
> otherwise.

I was thinking along the lines of how a ferromagnetic core in a solenoid /
transformer will _accentuate_ the magnetic field and extrapolating that
back to the steel next to floppy disk setup.  

> > Anyway, AIUI, the X-ray machines don't generate magnetic fields so that
> > shouldn't be a problem at all.
> 
> no, but the X ray machines produce photons which can't pass through the 
> steel, they get absorbed.  The X ray thing is entirely seperate to the
> not-being-wiped-by-magnets thing, sorry I didn't make this clear.

I was assuming the disks would only get exposed to X-rays (from the
security checking viewpoint).  Were you thinking of explicit magnetic
scanning of post or just accidental exposure in the first place?

Conor
-- 
Conor Daly <conor.daly at oceanfree.net>

Domestic Sysadmin :-)
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