[thedoc (OP)]

Magnetic Fingerprinting - see the data which is stored on the magnetic strip of your credit card.

Read more about this kitchen science experiment.

[chapter podcast=2823 track=10.11.01/Naked_Scientists_Show_10.10.31_7441.mp3] Listen to the Experiment[/chapter] or [download as MP3]

[daveshorts]

We were sent some lovely closeup photos form Randy Hirsch
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[wolfekeeper]

I was trying to work out if I could actually find a way to read off the magnetic poles. The problem with iron is that it sticks either way around (it's a soft magnetic material).

I figure if I could crunch up a ferrite magnet, put the crunched up bits on another magnet to orient them, and then paint one side, and then break it up again, it might be possible to read the poles by visual means.

I haven't tried it yet though. It might be fiddly to get the exact recipe right.

[SeanB]

The actual polarity is not relevant, as it is the changes that are read out by the read head and electronics. The actual polarity of individual magnetic domains is not important, the change is.

[wolfekeeper]

Depends what you want to use it for. I'm messing about with Halbach arrays and it would actually be useful to know polarities.

[SeanB]

Most card readers use a tape head, which is a basic coil with a gapped core, and follow it with a large amount of amplification that is AC coupled for stability. Thus the output is a differentiated representation of the magnetic recording, and pretty much all polarity information is stripped out when it is converted to a digital pulse train by a fixed point comparator. You get ( on the lowest data rate tracks) a clock and data that is robust and easy to recover, but which is not very high density recording.

Magnetic card media specifications allows a card to be paper with a screen printed stripe, something that is cheap and robust, but able to hold enough information to allow it to be used as a disposable parking token, or a more robust plastic laminated card that permanent parkers can use interchangeably on the machines.