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.

Features:

1. The contacts are enclosed, so oxidation of the contact points is impossible.
2. In hazardous locations, interrupting the circuit does not emit a spark that could ignite flammable gases.
3. Contacts stay clean, and even if an internal arc occurs, the contact surfaces renew on every operation, so they don’t wear out.
4. Even a small drop of mercury has low resistance, so switches can carry useful amounts of current in a small size.
5. 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.
6. The switches are quiet, as no contacts abruptly snap together.
7. The mass of the moving mercury drop provides an over-center effect to avoid chattering as the switch tilts.