Mercury Switch

A mercury switch (also known as a mercury tilt switch) is a switch whose purpose is to allow or interrupt the flow of electric current in an electrical circuit in a manner that is dependent on the switch's physical position or alignment relative to the direction of the "pull" of earth's gravity, or other inertia.

Mercury switches consist of one or more sets of electrical contacts in a sealed glass envelope which contains a bead of mercury. The envelope may also contain air, an inert gas, or a vacuum. Gravity is constantly pulling 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 the electrical circuit through those contacts. Tilting the switch the opposite direction causes the mercury to move away from that set of contacts, thus breaking that circuit. The switch may contain multiple sets of contacts, closing different sets at different angles allowing, for example, single-pole, double-throw (SPDT) operation.

Advantages of the mercury switch over other types are that the contacts are enclosed, so oxidation of the contact points is unlikely; in hazardous locations, interrupting the circuit will not emit a spark that can ignite flammable gasses. Contacts stay clean, and even if an internal arc is produced, the contact surfaces are renewed on every operation, so the contacts don't wear out. Even a small drop of mercury has a low resistance, so switches can carry useful amounts of current in a small size. The 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 there are no contacts that abruptly snap together. The mass of the moving mercury drop can provide an "over center" effect to avoid chattering as the switch is tilted. Multiple contacts can be included in the envelope for two or more circuits.

Disadvantages when compared with other types include: Mercury switches have a relatively slow operating rate due to the inertia of the mercury drop, so they are not used when many operating cycles are required per second. Mercury switches are sensitive to gravity so may be unsuitable in portable or mobile devices that can change orientation or that vibrate. Mercury compounds are highly toxic and accumulate in any food chain, so mercury is not permitted in many new designs. Glass envelopes and wire electrodes may be fragile and require flexible leads to prevent damaging the envelope. The mercury drop forms a common electrode, so circuits are not reliably isolated from each other if a multipole switch is used.