reed switch
How a magnetic reed switch works.
More pictures of reed switches

How a magnetic reed switch works

  

  

A reed switch consists of two or three springy metal reeds having plated, long-life contacts at the tips and encapsulated in a sealed glass tube. The two-reed type has normally open (NO) contacts which close when operated, and the three-reed type is a changeover, i.e. it has a pair of normally open (NO) and a pair of normally closed (NC) contacts. Reed switches are actuated by the field from an external permanent magnet or electromagnet placed in close proximity.

This field causes the reeds to become magnetic, the ends are attracted and the contacts either open or close. Removal of the magnetic field allows the springy reeds to restore the contacts to their original state. Reed switches have only a momentary action - their contacts revert back to their original state as soon as the magnetic field diminishes. Ref. Reed Switches by James Johnston

Note that reed switches are fragile, low power devices!

reed switch
Another reed switch with NO/NC contacts. (SPDT)

reed switch turning on triac

Closing the reed switch will cut on the triac. The idea is to use a small low-power switch to control high power devices such as motors or heaters. The danger is here is the high voltage AC is on the switch.