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The Watt steam governor is a device for regulating the speed of steam engines. It was invented in 1788 by James Watt, as the name implies. The device works in a rather simple way, but it's a bit hard to describe. I'll try:

The device is comprised of two metal balls attached to the top of a vertical rod by a hinged arm each. The rod is connected to the steam engine. When the rod rotates, the metal balls and arms start to rise because of centrifugal forces. The hinged arms are connected to the steam valve that feeds the engine, so when the engine speed exceeds a certain limit, the arms rise too high and the steam starts to get shut off. This reduces the speed of the engine, causing the balls to go down and the valve to open again. Thus the engine's speed does not exceed a certain level. The Steam Governor thus acts as a a negative feedback system.

The main problem with this system is that the engine speed will not decrease immediately. The engine will thus slow down too much, prompting a subsequent rise in speed. This results in a certain amount of oscillation. With most applications this isn't a problem though.

You can look at a picture of one at http://www.naaidt.org.uk/resources/milestones/getimage.html?ImageDirectoryNumber=Image0&Id=3&DontCache=67712&All=3

Sources:
http://www.naaidt.org.uk/resources/milestones/record.html?Id=3&All=3
http://www.gyford.com/phil/uhcl/systems/examples.html
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