Many office buildings have scanners that give access to the building. Many offices are switching to personal scanners that are unique to the individual. Some offices even have scanners in different parts of the building that can "read" where the employee is in the building. Disconcerting for purposes of civil liberties perhaps but could be useful if dealing with alibi or surveillance issues. Check out this excerpt from article from "How Stuff Works".
Security passcards are often used to gain entry into areas and buildings with restricted access. The security passcard may be for general access, meaning that the passcard does not provide data about the person using it, or it may be individually encoded, containing specific information about the cardholder.
Typically, the data on an encoded security passcard includes:
* ID # (social security number or other unique number)
* Access level (where you're allowed to go)
An individually-encoded passcard looks a lot like a credit card. The stripe on the back of the security passcard 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 on the back of the card is very similar to a piece of cassette tape.
The magstripe can be "written" because the tiny bar magnets can be magnetized in either a north- or south-pole direction. When the polarity of the bars aligns in the same direction, the card is blank. To write data requires a process called flux reversal. Basically, the polarity of a bar is reversed so that the north pole is facing the north pole of the adjacent bar (N-N) or the south pole is facing the south pole (S-S). This causes a change in the magnetic field that can be detected by the card reader. Since there can be two different flux reversals, N-N or S-S, there can be two different information states. This corresponds nicely to the binary system of 1s and 0s used by computers.