contacts abruptly snap together. The mass of the moving mercury drop provides an over center effect to avoid chattering as the switch tilts. The envelope can include contacts for two or more circuits. Disadvantages Mercury switches have several disadvantages: There is a tendency for the intermittently exposed electrode to become damaged by the intense heat and destructive force of the electrical arc that forms whenever the circuit opens or closes, particularly if the circuit is being opened under a large inductive load. Certain refractory materials have been used to encase this electrode to mitigate this effect Their relatively slow operating rate (due to the inertia of the mercury drop) makes them unsuitable for applications that require many operating cycles per second. Glass envelopes and wire electrodes may be fragile and require flexible leads to prevent damage to the envelope. The mercury drop forms a common electrode, so circuits are not isolated from each other in a multi-pole switch. Their sensitivity to gravity may make them unsuitable in portable or mobile devices that can change orientation or vibrate. Mercury compounds are highly toxic and accumulate in any food chain, so safety codes exclude mercury in many new designs. Uses Roll sensing Tilt switches provide a rollover or tip over warning for applications like construction equipment and lift vehicles that operate in rugged terrain. There are several non-mercury types, but few are implemented due to sensitivity to shock and vibration, causing false tripping. However, devices resistant to shock and vibration do exist. Automotive uses Automobile manufacturers once used mercury switches for lighting controls (for example, trunk lid lights), ride control, and anti-lock braking systems. Scrapped automobiles can leak mercury to the environment if these switches are not properly removed. Since 2003, new American-built cars no longer use mercury switches.Organisation for Economic Co-operation and Development, Instrument mixes for environmental policy OECD Publishing, 2007,, pg.145 Fall alarms Work performed in confined space (such as a welder inside a tank) raises special safety concerns. Tilt switches sound an alarm if a worker falls over. Aircraft attitude indicators/artificial horizons Electrically driven attitude indicators typically use mercury switches to keep the gyro axis vertical. When the gyro is off vertical, mercury switches trigger torque motors that move the gyro position back to the correct position. (Air driven attitude indicators use a different operating principle.) Thermostats Mercury switches were once common in bimetal thermostats. The weight of the