dead man's switch

A dead man's switch is a switch that is designed to be activated or deactivated if the human operator becomes incapacitated, such as through death, loss of consciousness, or being bodily removed from control. Originally applied to switches on a vehicle or machine, it has since come to be used to describe other intangible uses, as in computer software. These switches are usually used as a form of fail-safe where they stop a machine with no operator from a potentially dangerous action or incapacitate a device as a result of accident, malfunction, or misuse. They are common in such applications in locomotives, aircraft refuelling, freight elevators, lawn mowers, tractors, personal watercraft, outboard motors, chainsaws, snowblowers, treadmills, snowmobiles, amusement rides, and many medical imaging devices. On some machines, these switches merely bring the machines back to a safe state, such as reducing the throttle to idle or applying brakes while leaving the machines still running and ready to resume normal operation once control is reestablished. Dead man's switches are not always used to stop machines and prevent harm; such switches can also be used as a fail-deadly, since a spring-operated switch can be used to complete a circuit, not only to break it. This allows a dead man's switch to be used to activate a harmful device, such as a bomb or improvised explosive device. The switch that arms the device is only kept in its "off" position by continued pressure from the user's hand. The device will activate when the switch is released, so that if the user is knocked out or killed while holding the switch, the bomb will detonate. The Special Weapons Emergency Separation System is an application of this concept in the field of nuclear weapons. A more extreme version is Russia's Dead Hand program, which allows for automatic launch of nuclear missiles should a number of conditions be met, even if all Russian leadership were to be killed. A similar concept is the handwritten letters of last resort from the Prime Minister of the United Kingdom to the commanding officers of the four British ballistic missile submarines. They contain orders on what action to take if the British government is destroyed in a nuclear attack. After a prime minister leaves office the letters are destroyed unopened.