Now you know

Why must we 'remove card quickly' when we swipe a credit card at the gas pump? The black strip on the back of the card is magnetic. It contains magnetic particles 20 millionths of an inch long. The particles have been oriented so they lie in one of two directions. The resulting binary code contains your card number and other data. When a magnet moves past a wire, it generates an electrical current in the wire. The faster the magnet moves, the stronger the electrical current generated in the 'read head' at the pump. How fast is 'quickly'? The devices can read cards moving from 3 to 50 inches per second (34 miles per hour). 'Steady' is more important than 'quick.' But a quick swipe tends to be steady, too.

Source: 'Do Elephants Jump?' by David Feldman (HarperCollins 2004)