The stripe on the back of a credit card is a magnetic stripe,
often called a magstripe. The magstripe is made up of tiny iron-
based magnetic particles in a plastic-like film. Each particle
is really a very tiny bar magnet about 20 millionths of an inch
long. The magstripe can be "written" because the tiny bar
magnets can be magnetized in either a north or south pole
direction. The magstripe on the back of the card is very similar
to a piece of cassette tape fastened to the back of a card. (See
How Tape Recorders Work.) Instead of motors moving the tape so
it can be read, your hand provides the motion as you "swipe" a
credit card through a reader or insert it in a reader at the gas
station pump. On the next page, see how information is stored in
the mag stripe and read by different types of machines.