A Tilt Switch is a device used which gives out a signal at a pre-defined tilt angle. Originally, tilt switches were made of Mercury, but due to Mercury being a hazardous substance, Reed Switch based Tilt switches have been developed. A Reed Switch based Tilt Switch consists of a reed switch a moving magnet assembly in a single package, that is designed to actuate at a set angle.
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.