Hermetically sealed reed switches
The hermetically sealed reed switch is a very common type of two piece sensor that operates with an electrically conductive reed switch that is either normally open or normally closed when under the influence of a magnetic field as in the case of proximity to the second piece which contains a magnet. When the magnet is moved away from the reed switch, the reed switch either closes or opens, again based on whether or not the design is normally open or normally closed. This action coupled with an electric current (typically at 12 V DC) allows an alarm control panel to detect a fault on that zone or circuit. These type of sensors are very common and are found either wired directly to an alarm control panel, or they can typically be found in wireless door/window contacts as sub-components.