Abstract: A mercury switch is provided in a cartridge for insertion into a relay having a plurality of flux fingers spaced apart to define a flux finger gap, the cartridge being received in the relay and bridging the flux finger gap. The mercury switch includes a pair of mercury layers having an air gap therebetween, the mercury layers merging in response to a magnetic field to form a continuous electrical conductor. The mercury switch is disposed across the width of the cartridge in the flux finger gap. Flux plates are disposed within the cartridge and magnetically coupled to the switch to provide a low reluctance path between the flux fingers, the path including the switch air gap. The switch may be operated in a normally open position; or with the insertion of a bias magnet in the cartridge, the switch may be operated in a normally closed position.