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A dead man's switch (for other names, see alternative names) is a switch that is automatically operated 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 like in computer software. These switches are usually used as a form of fail-safe where they stop a machine with no operator from 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, tread machines, 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. When the switch is an electrical one, it is usually wired as part of a series circuit. Dead man's switches are not always used to stop machines and prevent harm. These switches can also be used as a fail-deadly. A spring-operated switch can also be used to complete a circuit when it is no longer held down. This means that a dead man's switch may be used to activate a harmful device, such as a bomb or IED. The user holds down a switch of some sort in their hand which arms the device. The device will activate when the switch is released, so that if the user is 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 has been employed with computer data, where sensitive information has been previously encrypted and released to the public, and the "switch" is the release of the decryption key, as with the WikiLeaks "Insurance File". ==Background== Interest in dead-man's controls increased with the introduction of electric streetcars and especially electrified rapid transit trains, though dead-man equipment was quite rare on US streetcars until the successful PCC streetcar, which had a left-foot operated dead-man's pedal in conjunction with the right-foot operated brake and power pedals. This layout has continued to be used on some modern trams around the world. In conventional steam railroad trains, there was always a second person with the engineer, the ''fireman'', who could almost always bring the train to a stop if necessary. For many decades this practice continued on electric and diesel locomotives, even though a single person could theoretically operate them. With modern urban and suburban railway systems, the driver is typically alone in an enclosed cab. Automatic devices were already beginning to be deployed on newer installations of the New York City Subway system in the early 20th century. The Malbone Street Wreck on the Brooklyn Rapid Transit system in 1918, though not caused by driver incapacitation, did spur the need for universal deployment of such devices to halt trains in the event of the operator's disability. According to a Manhattan borough historian, there have been at least three instances where the dead-man's switch was used successfully – in 1927, 1940, and 2010. The status and operation of both vigilance and dead-man's switch may be recorded on the train's event recorder (commonly known as a ''black box''). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Dead man's switch」の詳細全文を読む スポンサード リンク
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