Magnetic Force

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15 Responses:

  1. Wout says:

    Good thing magnetic stripe credit cards are on the way out... I imagine visitors don't even have to come close to erase theirs.

    • jwz says:

      It's a lot harder to erase a card or tape than you think.

      • Wout says:

        Hmm, I have a WayTools Bluetooth "origami" keyboard that folds down to the size of a snack bar, I carry it with me everywhere.
        It uses magnets throughout, and I have definitively wiped several hotel keycards by putting them in my pocket with it.

        Of course, that is close contact, and probably that cube didn't get a really high magnetic field embedded at manufacture. Hmmm.

        • Zygo says:

          Hotel keycards--and hotel keycard readers--live right between the noise floor and lowest-bidder economics. The readers often run on half-dead batteries and can barely read freshly written cards, much less lightly stressed ones.

          • Wout says:

            Interesting, never considered that. Makes lots of sense.

            Reminds me of the time I was playing with a speaker magnet and a cassette in the 80s and by moving the magnet across the tape once or twice, the tape was full of echos of itself.

      • margaret says:

        I used to hold my HD floppies on the refrigerator with rare-earth magnets. Never once detected an error.

        • Wout says:

          And that makes some sense in retrospect, since the signal is so small to begin with and it's all encoded as relative differences, so the bulk magnetization won't impact that much? Or recording is done with a material that is highly resistant to magnetic change until it's strong enough?
          Not familiar with digital recording technology.

          Cassette audio was just an AM signal apparently, and that tape I echoed was cheaply made.

          Pretty hardcore way to store floppies in any case!

          • margaret says:

            It did get everyone's attention, particularly the one marked "THESIS - Final." I learned this trick when I tried to deliberately erase a floppy w/o putting it in the computer. The bits won. No idea about how or why, only that conventional wisdom on the topic was all wrong.

            • Jieves says:

              When I used to work the late shift as a student computer center supervisor at a college in the Northeast I always found that students moving from their steam-heated dorm rooms through the snow and then back into the steam heat of the Computer Center was much more damaging to floppy disks than magnets ever were. Norton Utilities and I became great friends around thesis time.

          • Glaurung says:

            The coercivity (in oersteds) of magnetic media varies widely. To disrupt the data on the media, you need a magnet with a gauss strength 2-3 times the coercivity of the media.

            Analog tape has a coercivity of 300-400 oersteds. Floppy disks have a coercivity of 700. Hard disks have a coercivity of several thousand.

            Source (from 2004, hard disks have probably gotten harder to erase since then): http://www.akl-it.com/manual/Media%20gauss%20level%20whitePaper_ABCofDegaussing.pdf

            Also, from experience, I know that a stationary magnet is less damaging to a floppy than a moving magnet. Sticking it to the fridge is one thing, swirling the disk around while it's touching the magnet is another.

            • thielges says:

              > from experience, I know that a stationary magnet is less damaging to a floppy than a moving magnet.

              Your experience matches the science. A changing magnetic field is required to flip stored magnetic states. A static magnetic field won’t do squat, even an enormously strong field. Degaussing tools are generally made by running an AC current through a coil to generate that second derivative in the magnetic field.

              • tfb says:

                I remember watching a Mac (SE? SE30?) near the ALEPH detector of LEP as they tested the magnets. The screen became twisted well beyond readability, but the machine managed fine. Disks turn out to be pretty tough.

                Listening to the liquid helium boil and vent to the atmosphere when the magnets quenched is something I will remember forever.

                • thielges says:

                  Yeah, ALEPH certainly qualifies as a massive magnet. There’s still two pulses of changing fields when it is powered up and down though presumably something in the device moderates the ramp up and down slopes to avoid generating destructive pulses.

                  Surprised to hear that helium was just vented into the air instead of being recycled. Please tell me that at least the scientists in the room would use it sometimes for Donald Duck voice merriment.

                  • tfb says:

                    I think in production the helium was going to be recycled: when I saw it (1988?) they were working on getting it all working and there were bits not yet finished. I also remember that quenches were pretty bad for it (they were certainly very noisy: a big bang as circuit-breakers tripped then a pause, then this huge rising scream of gas boiling off) and it could only withstand n of them, though I don't know what n was.

  2. internetimal says:

    Great magnet!

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