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Physics of mag tapes

   Date: Sat, 16 Sep 89 13:33:55 PDT
   From: felix@warbucks.ai.sri.com (Francois Felix INGRAND)

   Paul Pangaro writes:
   >  I have been taking cart tapes for SMBX in and out of airport
   >  security for years now, and sometimes with the same tape a dozen
   >  times. Coming back from Boston recently I was met with shock from
   >  the security-person that I was doing this, because "everybody
   >  else passes them through by hand" and not through the x-ray
   >  machine.

   I am not sure that X-ray will damage your magnetic tapes... I had
   rather to put them in the x-ray machine than carrying them with me
   thru the metallic "door frame" detector.

   Of course, it is the opposite for photographic films...

   >  Anyone really know what those things do, and whether it can, over
   >  repeated exposures, affect the oxide/fields on the tape?  I
   >  realize "it is probably better to be safe than sorry" but I was
   >  curious about the known realities here.

   If you look on the written notice coming with the tapes or the
   floppydisks, they mention magnetic field as a potential hazard, but
   not x-ray.

   >  Best,
   >  PANgaro


X-rays are simply an energetic form of light, and should no
more harm any magnetic media than an ordinary flashlight.

As for the magnetic field of the machine, because magnetic force
drops off very quickly with distance, if there is at least
a few inches of "air-space" around a tape or disk, only
extremely powerful magnetic fields can erase it. I read an
article once stating that if a tape or disk is in a box
with a few inches of foam padding on all sides, even a
bulk-eraser passed around the outside of the box won't harm it.

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