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You used to be able to buy magnetic field viewers to inspect tape. Here's a picture of one: http://nickapedia.com/2011/02/21/back-to-the-geek-future-coo...

Another fun thing you can do is put the iron filings in a jar with mineral oil. Give it a good shake, then put a strong magnet on the side. You'll be able to see the magnetic field in 3D.




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