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The magnetic stripe on a credit card is very much like a piece of magnetic recording tape. It is written and read in much the same way that audio recording tape is written and read, except that the data are binary digits --- 1s and 0s --- instead of sounds. Companies that deal with these cards have agreed among themselves how to use arrangements 1s and 0s to represent the letters and numbers that they want to store on the stripe.
The magnetic stripe is made of a material that can be magnetized. To write data onto the stripe, the card is dragged over a tiny electromagnet. Pulses of electrical current are pushed through the electromagnet windings in one direction or the other, to magnetize tiny spots on the stripe material. Perhaps one direction of magnetization represents a "1", while the opposite direction represents a "0"; but more likely a slightly more complicated system is used. To read the data, the card is dragged over a tiny coil of wire. Movement of the magnetized spots past the coil causes small electrical voltages to appear in the coil, and from these voltages the stored 1s and 0s can be deduced.
In practice, the coil used to read the stripe is very similar to the electromagnet used to write the stripe. These electromagnets are mounted in "write heads" and "read heads", which typically hold not one but several electromagnets, side by side, so that several "tracks" of magnetized spots are written and read simultaneously.
The FAQ of a popular hacker's group gives the following description of the layout of the data on the stripe. The abbreviation "bpi" stands for "bits per inch," meaning the number of 0s or 1s that one head can write on each inch of stripe.
3. What is the layout of data on magnetic stripe cards? A standard card may have any of three tracks, or a combination of these tracks. Track 1 was the first track standardized. It was developed by the International Air Transportation Association (IATA) and is still reserved for their use. It is 210 bpi with room for 79 characters. It includes the primary account number (up to 18 digits) and the name (up to 26 alphanumeric characters). Track 2 was developed by the American Bankers Association (ABA) for on-line fincancial transactions. It is 75bpi with room for 40 numeric characters. It includes the account number (up to 19 digits). Track 3 is also used for financial transactions. The difference is its read/write ability. It is 210bpi with room for 107 numeric digits. It includes an enciphered PIN, country code, currency units, amount authorized, subsidiary account information and other restrictions. For more information, read the ANSI/ISO 7811/1-5 standard. This document is available from the American Bankers Association.
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