movable mercury drop provided some hysteresis by a degree of over-center action. The bimetal spring had to move further to overcome the weight of the mercury, tending to hold it in the open or closed position. The mercury also provided positive on-off switching, and could withstand millions of cycles without contact degradation. Doorbells Some old doorbells, for example, the Soviet ZM-1U4, use mercury switches as current interrupters. Pressure switches Some pressure switches use a Bourdon tube and a mercury switch. The small force generated by the tube reliably operates the switch. Vending Mercury switches are still used in electro-mechanical systems where physical orientation of actuators or rotors is a factor. They are also commonly used in vending machines for tilt alarms'' that detect when someone tries to rock or tilt the machine to make it vend a product. Bombs A tilt switch can trigger a bomb. Mercury tilt switches can be found in some bomb and landmine fuzes, typically in the form of anti-handling devices, for example, a variant of the VS-50 mine. Toxicity Since mercury is a toxic heavy metal, devices containing mercury switches must be treated as hazardous waste for disposal. Because it is now RoHS restricted, most modern applications have eliminated it. A metal ball and contact wires can directly replace it, but may require additional circuitry to eliminate switch bounce. Low-precision thermostats use a bimetal strip and a switch contact. Precision thermostats use a thermistor or silicon temperature sensor. Low-cost accelerometers replace the mercury tilt switch in precision applications. In the United States, the Environmental Protection Agency (EPA) regulates the disposition and release of mercury. Individual states and localities may enact further regulations on the use or disposition of mercury. See also Mercury-arc valve, a rectifier device intended for high electrical voltages/currents Mercury battery, an electrochemical battery Mercury coulometer, an electro analytical chemistry device that determines the amount of matter transformed during a mercury reaction Mercury probe, an electrical probing device to sample for electrical characterization Mercury swivel commutator, an electrical circuit, current-reversing switch using the element mercury Mercury-wetted relay Mercury relay References External links How to Adapt a Mercury Switch for Robot Control Heating, ventilation, and air conditioning Switches Mercury (element) fr:Interrupteur#Mercure