Re: Platinum 5.25" drive problem

[mjmahon@aol.com (Michael J. Mahon)]

Matthew Russotto wrote (in response to me):

>In article <20010710181837.03517.00004552@ng-fi1.aol.com>,
>Michael J. Mahon <mjmahon@aol.com> wrote:
>>
>>In short, don't worry about using permanent magnets to disrupt the
>>magnetic patterns recorded on floppys (but don't magnetize the hub
>>of a 3.5" diskette--that's not good.  ;-).
>
>You can't damage the head with a magentized floppy.  But you can make
>the floppy unusable until demagnetized.  Bulk erasers use an AC field,
>and attenuate it gradually at the end of the cycle.

The erase and write heads of the floppy drive are, by definition, strong
enough to deal with magnetic media in ANY state of magnetization.

As you point out, the critical requirement for DE-magnetization is the
gradual reduction of an AC field which is, initially, strong enough to
overcome the coercivity of the magnetic media.  As the field strength
is reduced, the remanent magnetization of the media is "walked down"
the hysteresis loop until a minimal (never zero) level of net magnetization
remains.

Whether a CRT degaussing coil produces a strong enough field to
affect a floppy depends greatly on 1) the peak local strength of the
degaussing coil at the point where you hold the floppy, and 2) the
coercivity of the floppy media.  (HD disks are much harder to "flip"
than DD disks.)

I would not depend on a monitor degausser to erase a floppy, but
neither would I hold a floppy I cared about near one during the
degauss cycle.  ;-)

-michael

 Email:  mjmahon@aol.com
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