The Rule #29-39 ECO-Switch is an ecologically sound automatic bilge pump control control switch. Featuring a non-mercury design, the pump operates with simple air pressure. As water rises in the bilge, air rises in the air tube that is attached to switch. This air pressure activates the switch which controls your pump. There are no moving internal parts, all wiring is out of the water, and there are no submerged electrical connections at all. Offering complete reliability and outstanding performance, the Rule ECO-Switch offers a viable alternative to traditional float switches.