Re: World-wide 8-bit computer sales (all-time)?

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

Randy McLaughlin wrote:

>"Wayne Stewart" <waynes@telus.dotnet> wrote in message
>news:3F0248D2.90402@telus.dotnet...
>>
>>
>> Sam Gillett wrote:
>>
>> > IMHO, the drives in a Mac have enough shielding to protect data from any
>> > degaussing that occurs at power-on time.
>> >
>> > In the case of a monitor sitting on top of a desktop unit, the metal
>case of
>> > the computer provides additional shielding.
>> >
>> > But, it is still a bad idea to sit a box of disks on top of your color
>> > monitor.  Even if that monitor is attached to a C64 or other classic
>home
>> > computer.
>>
>> When I started with computers I read about all these things you
>> shouldn't do. But I always liked to experiment. Keep disks away
>> from magnets -- stuck one to the fridge with a big magnet for a
>> couple of years, still worked fine. Keep disks away from monitors
>> -- had some tight against one for several years, still worked.
>> Don't put your disks on an Apple Graphics tablet -- that wiped the
>> disks clean!!
>>
>> Wayne
>
>A static magnetic field may not erase a disk it is a flux that alters
>(erases) the disk.  Add to that the data on the disk is recorded on very
>small pieces of iron, some disks had larger iron pieces and required more
>energy to charge them (write/erase).

Incorrect.  Any sufficiently strong magnetic field will re-magnetize
the magnetic domains of the media, all but eliminating any useful
remanent of the original, information-bearing magnetization.

To reduce the remanent magnetization to zero, an alternating
field of sufficient strength to overcome the coercivity of the medium
is required, whose strength is reduced gradually to zero.  This
process "walks" the domains down their hysteresis loops to
near-zero magnetization.

It is not necessary to have zero magnetization to obliterate the
original magnetization patterns.  DC and permanent magnet fields
have been used for years to erase magnetic media.  (Though,
of course, they do not leave them demagnetized--merely re-
magnetized in a non-information-bearing configuration.)

-michael

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