Here's a fun science experiment: finely powder some rust and then blow it over the magstripe on your credit card and you can see the zeroes and ones encoded on it by the stripes where the magnetic forces attract the ferrous particles. For a bonus, Anaglyph tried this out on a woo-woo product called a Shoo!Tag, which is supposed to use "a three dimensional or trivector signature imprinted onto the magnetic field of a three field magnetic memory card to create a protective barrier from pests." No evidence of a "trivector signature" was in found.
Amazing! The fine particles clearly delineate the data on the card! What we're seeing here tells us lots about how a credit card works. First of all, you will notice that Gilbert's card has three horizontal magnetic bands. This is the standard for all swipe cards. In most cases, information is recorded on one, or sometimes two of these bands. The two outside bands are called high density tracks and contain data at 210 bits per inch. If you know anything about computers, you will realise that the term 'high density' here is relative: 210 bits per inch, by modern data standards, is pretty damn lousy. To give you some idea, one of these tracks can carry about 79 x 6bit alphanumeric characters. Your credit card would typically have, on track 1, your name, your card number and an expiry date. That's it. Not much.