|Re: 41 vs 67 card reading|
Message #5 Posted by Bernhard on 25 Oct 2005, 4:31 p.m.,
in response to message #4 by Etienne Victoria
I've ***never*** seen a HP65 or HP67 card fail due to "degaussing".
Neither I have never seen any diskette fail due to "degaussing".
And believe me - I have a lot. Including 25 year old disks and even older cards. I do even have 30 year old 8" disks that still read OK.
The physics behind this is hysteresis of the magnetic particles. It takes a certain field strength to flip them. And even if one particle is flipped by thermodynamic accident, zillions of other particles still wait for the same strong field to be flipped. The HD (high density) media used in the PC even takes much much higher fields to flip. They are almost impossible to erase of write to in old Apple II disk drives, for instance, because the magnetic flux of those drives is too low to flip them.
The bottom line is, in magnetic media there is no effect like "degaussing" - if this means flipping of magnetic domains just by ageing without strong external magnetic fields.
What really happens is that the softening agents gass out and the magnetic particles start to separate from the plastic carrier. This is not "degaussing" but "drop out".
I do have some HP65 / HP67 cards that won't read anymore, but under magnification, it is possible to see those damage spots where the magnetic particles have gone away.
The only cure is to COPY you old cards to newer cards. But do do try to "refresh" the old cards - it is futile - as long as they read OK and don't fall apart mechanically (as described above) they are good. And if they start to lose magnetic coating, well, the they are finished and no attempt to write on them will fix that.