In brief

The reed switch is based on two principles: buoyancy and magnetism.
Buoyancy causes the float (which contains a magnet) to rise up or descend along with the liquid. The end of the pivot arm (fixed part of the instrument) contains a permanent magnet that can repel the switch magnet (inside the fixed structure).
When the float magnet approaches the fixed rod of the switch the float magnet repels the switch magnet that opens or closes the electrical circuit. Applications include level control in tanks in the marine, chemical, wastewater treatment, and water industries.