The first reed switch came into being in 1936. It was an invention of W. B. Elwood at Bell Telephone Laboratories. It is a specialized type of switch that operates magnetically. This switch has the ability to trigger a siren when motion is detected as part of a burglar alarm. It is also commonly used with electrical circuit controls and is an important device in the communications industry.
A reed switch consists of a pair of ferrous metal contacts or three springy metal reeds located inside a sealed glass tube. The two-reed switch normally has open contacts that are closed while in operation, referred to as (NO) contacts. The three-reed type, on the other hand, has a pair of (NO) and a pair of normally closed contacts, which are referred to as (NC).
The switch is enclosed in a small tube, which is controlled by a magnet or magnetic field. It works with the help of an additional magnet, which is placed within close range of the switch. When the magnets make contact, they pull together and complete an electrical circuit. This results in a change in the contacts to an opposite state.
As the magnetic field diminishes, the switch and its contacts immediately return to their original state. A magnetically operated reed switch such as this is commonly mounted on doors and windows to add protection to a home or business. The switch is small and discreet enough to mount in a home or office without being noticeable.
While the small size of the reed switch makes it easy to mount and to operate, there are some disadvantages. For instance, because it is small and delicate, it cannot maintain a large voltage of current. Instead, the current may simply pass right through. In turn, excess voltage can cause the switch to spark when in use. It is also known to overheat when used with a heavy current. This can cause the switch to lose its spring-like form.