Reed switches are magnetically actuated switches. They are typically manufactured with two ferromagnetic reeds (contact blades), which are sealed in a glass capsule. In the presence of a magnet, the blades (contacts) close.  Reed switch types are dry reed switch or mercury wetted reed switch.  A dry reed switch contains ferromagnetic contact blades sealed in a glass container with an inert gas.  In a mercury wetted reed switch, mercury is contact material for electrical circuit, mercury to mercury. The contacting faces are renewed by capillary action drawing a film of mercury over the surfaces of the constant switching members as the movable contact member is moved from one position to another.  The center position on reed switches can be center gap or off center gap.  The gap is the contact meeting of the switch.