How does a reed switch work ?
Flux density produced in a test coil, or produced at the reed switch position by a magnet, becomes the coercive force, “H”, to create magnetic induction, “Bi”, in the permeable reeds. The field induced in the reeds adds to the magnetizing field strength (coercive force) to develop field intensity in the gap between the reeds. As the gap flux density increases, the magnetic attraction between the reeds causes them to bend and close. An open reed switch has two short, low permeance parts, and when the switch closes, the reeds join to become one high permeance part. This is why the “pull-in” rating (required flux density), is greater than the “drop-out” rating.
When a magnetic force is generated parallel to the reed switch, the reeds become flux carriers in the magnetic circuit. The overlapping ends of the reeds become opposite magnetic poles, which attract each other. If the magnetic force between the poles is strong enough to overcome the restoring force of the reeds, the reeds will be drawn together.
