Reed is dry reed short, is a kind of passive electronic switch contact element has a simple structure, small size and easy to control. The shell is in a sealed glass tubes in which there are two flexible iron reed panels, also called metal rhodium has a filling of inert gas. Normally, two glass tubes made of special materials reeds are separated. When a magnetic substance near the glass tube, under the action of magnetic field lines, the two reed pipe is magnetized and attract each other contacts, the reed will pull together to make a circuit of the connected node communication. After the disappearance of the external magnetic force, two reeds separately because of their elasticity, the line also disconnected. Therefore, as a signal to control the use of magnetic field lines switching devices, the reed can be used as a sensor for counting, spacing, and so on (in the security system is mainly used for door sensor, window magnetic production), but also It is widely used in a variety of communication devices. In practice, usually with a permanent magnet control the two metal sheets turned on or not, it is also known as "magnetron."