As machines get more complicated, quick-acting and violent, they are more prone to self-destruction if something goes wrong. Some nuclear reactors, for instance, can turn into radioactive junk in a fraction of a second. To avoid such misadventures, most modern mechanical and electronic systems are equipped with built-in monitors that watch their operation and shut them down promptly at the first sign of trouble. But if a vacuum tube or relay in the monitor fails, the main machine is like a building whose night watchman has dropped dead. Trouble can start and get out of hand with no one to correct...
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