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A reed switch is a type of relay (or switch) that is controlled by a magnetic field. Two contact leads are hermetically sealed in a glass envelope. When a magnetic field is close enough, the two leads either make contact or break contact, depending on which type of reed switch you bought. This type of switch is used for proximity detection, because it provides a digital on/off function activated by the proximity of a magnet.
- Wikipedia, "Reed switch", http://en.wikipedia.org/wiki/Reed_switch