Biasing a Reed Switch with another permanent magnet will allow a normally closed (Form B) operation. Bringing a second magnet, of opposite polarity, in close proximity to the magnet/Reed Switch assembly will open the reed contacts.
Also, using a biasing magnet will allow the reed switch operation in the hold area or hysteresis area, thereby creating a latching sensor. In this situation, real care needs to be taken in exact placement of the biasing magnet and the operating magnet needs to be restricted to certain areas. To switch from bi-stable state to bi-stable state the operating magnet’s polarity or direction needs to be reversed.