The reed switch relies on two basic scientific principles namely: buoyancy and magnetism. Buoyancy causes the float (which contains a magnet) to rise with the liquid and magnetism helps open and close the switch.
A change in liquid levels raises or lowers the float up or down. The end of the pivot arm (non-float side) contains a permanent magnet that can repel the switch magnet (inside the stationary ‘stem’ of the entire structure).
When the float magnet moves close to the switch’s stationary stem, the float magnet repels the switch magnet which either opens or closes the Electrical circuit.