RFID-(ar ef i dee) - Radio Frequency Identification (sometimes Identification Device). A small device which can be contained in a glass capsule to be placed in pets, carried in cards, worn in clothing or a multitude of other places. Used by the US postal service (USPS) to track mail bags, etc.

These devices respond to a low-frequency radio field from a transmitter(usually lower than the AM radio band) storing energy until they have enough to send an id back to the transmitter. Thus they need no batteries, and are small enough to be contained in cards just barely thicker than a credit card, or buttons, or inside glass capsules the size of a large hypodermic needle.

Ford uses them in Dearborn, Detroit on their employee ID cards. Many employees carry them in their planners (or wallets), and simple pass their planners (or shimmy their behind-which is very peculiar to observe) close to the reader. Most RFID devices have ranges of 10-20 inches (25-50 cm) but can be further depending on the antenna and transmitter configuration. Typically the RFID antenna is so small that it has to be within several inches of the transmitter to work well, so don't get all paranoid about it, eh?

I want to attach one to my watch for my home automation system so the system can be more intelligent.