Magnetic Stripe Cards

A magnetic stripe card is a card (e.g., a credit card) that contains a stripe of magnetically-encoded data. These cards are paired with readers and writers, and are used in a wide variety of applications for storing information.

Usage

Most identification, credit, automated teller machine, and membership cards have a thin magnetic stripe across one side of the card. By "swiping" the card through or inserting the card into a reader, a computer can read the data on the stripe. Magnetic stripes make card usage much less prone to error because the data are not entered manually. The data cannot be read without a correctly programmed machine, and the cards must be created with special equipment, both of which make cards hard to falsify. The stripes can also contain encrypted information and information not printed on the card which can only be created or read by specialized computers.

Technologies

The magnetic stripe is actually an area of dense microscopic bar magnets, of which there are approximately 200 million per square inch. The magnets can be used to encode the data in an infinite number of ways. High security cards encode the data in proprietary, undisclosed formats that prevent intruders from even reading the data. However, all credit, banking, and automated teller machine cards use the same type of encoding. Most other cards use this encoding as well.

Like larger bar magnets, one end of each microscopic magnet is a north pole (N) and the other a south pole (S). In an empty, horizontal stripe, all of the bars are aligned left to right so that the north poles are facing one direction and the south poles the other. The north pole of each magnet is therefore adjacent to the south pole of the next magnet and, like other bar magnets, all of the aligned magnets act as one larger magnet.

Unlike usual bar magnets, these tiny magnets can be made to reverse their polarity. If a single or a continuous group of bar magnets is reversed, the stripe will act like three opposing bar magnets.

If several sections of magnets are reversed, the stripe looks like a series of opposing magnets.

The like poles of two magnets repel each other. This results in a strong magnetic field at those junctions. A magnetic stripe reader has a head, much like the one in an audio tape player, which can detect that field. The head senses the field at a north-north junction as a positive voltage and the field at a south-south junction as a negative one.

The reader converts the analog signal into a digital one by turning positive peaks into transitions from low to high and negative peaks into transitions from high to low.

Finally, the reader measures the length of each high/low pair. Each pair is interpreted as a bit—a short pair as a binary "1" and a long pair as a binary "0." In this way, the preceding sequence of magnets encodes the bits "110".

The difficulty in this system is that it depends on the reader measuring the distance between the peaks. The card is usually swiped through the reader by a person's hand, and it is difficult or impossible to regulate the specific speed at which the card should be swiped. At a low speed, a single high/low pair will look longer than it actually is. At a high speed, the same pair will look shorter. So, long pairs (zeroes) are exactly twice as long as short pairs (ones). In this way, no matter how rapidly or slowly the user moves the card, so long as the card is moved at a relatively constant speed, the machine reader can differentiate a one from a zero.

These bits are used to represent letters and numbers just as they are in a computer. On most cards, there are actually three independent stripes. The first stripe contains letters in the ANSI/ISO ALPHA format, and can contain a name or other text. The second and third stripes contain credit card or identification numbers, and are in binary coded decimal (BCD) format. While the first and second stripe contain "read-only" data, the third stripe is designed to be both written to and read while it is being swiped. This feature is rarely used, however.

These stripes can be made of several different materials, but can all be damaged by strong magnets. A strong magnet will align all of the magnets in the stripe, effectively erasing it.

In 2000 computer chips were added to some credit cards. Though these " smart cards " are read in the same way as cards with magnetic stripes— that is, they are swiped through readers—they are capable of holding substantially more information. These chips can also perform calculations, allowing the cards to act "intelligently."

see also Embedded Technology (Ubiquitous Computing); Input Devices; Security.

Salvatore Domenick Desiano

Bibliography

ANSI/ISO 7810-7816, Standards for Identification Cards. American National Standards Institute/International Organization for Standardization, 1987-1996.

O'Mahony, Donal, Michael Peirce, and Hitesh Tewari. Electronic Payment Systems for E-Commerce, 2nd ed. Boston: Artech House, 2001.

Rankl, Wolfgang, and Wolfgang Effing. Smart Card Handbook 2nd ed. New York: John Wiley & Sons, 2000.