[Techtalk] Sending floppy disks thru the mail

[Maria Blackmore]

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 :)

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.

It's really quite funky, you should try it, it's easy enough

You need some iron filings, a sheet of paper, a magnet, and a piece of
steel.
If you put the iron filings on the paper, they sit there.
If you then put the magnet under the paper, the iron filings make pretty
patterns.
If you put the paper on a piece of steel, and the put the magnet under the
steel, the iron filings don't make any patterns at all because the steel
has short circuited the field lines, neat huh?

> 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.


have fun :)

Maria