A mercury switch is an electrical switch that opens and closes a circuit when a small amount of the liquid metal mercury connects metal electrodes to close the circuit. There are several different basic designs (tilt, displacement, radial, etc.) but they all share the common design strength of non-eroding switch contacts. The most common is the mercury tilt switch. It is in one state (open or closed) when tilted one direction with respect to horizontal, and the other state when tilted the other direction. This is what older style thermostats used to turn a heater or air conditioner on or off. The mercury displacement switch uses a 'plunger' that dips into a pool of mercury, raising the level in the container to contact at least one electrode. This design is used in relays in industrial applications that need to switch high current loads frequently. These relays use electromagnetic coils to pull steel sleeves inside hermetically sealed containers. Description Mercury switches have one or more sets of electrical contacts in a sealed glass envelope that contains a small quantity of mercury. The envelope may also contain hydrogen at pressure, an inert gas, or a vacuum. Gravity constantly pulls the drop of mercury to the lowest point in the envelope. When the switch is tilted in the appropriate direction, the mercury touches a set of contacts, thus completing an electrical circuit. Tilting the switch in the opposite direction moves the mercury away from that set of contacts, breaking that circuit.<ref>{{cite book |first=Jacob |last=Fraden |title=Handbook of Modern Sensors - Physics, Designs and Applications (3rd Edition) |publisher=Springer - Verlag |year=2004 |pages= 256–257 }}</ref> The switch may contain multiple sets of contacts, closing different sets at different angles, allowing, for example, single-pole, double-throw (SPDT) operation. Advantages Mercury switches offer several advantages over other switch types: The contacts are enclosed, so oxidation of the contact points is impossible. In hazardous locations, interrupting the circuit does not emit a spark that could ignite flammable gases. Contacts stay clean, and even if an internal arc occurs, the contact surfaces renew on every operation, so they don't wear out. Even a small drop of mercury has low resistance, so switches can carry useful amounts of current in a small size. Sensitivity of the drop to gravity provides a unique sensing function, and lends itself to simple, low-force mechanisms for manual or automatic operation. The switches are quiet, as no