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A centrifugal governor is a specific type of governor that controls the speed of an engine by regulating the amount of fuel admitted, so as to maintain a near constant speed whatever the load or fuel supply conditions. It uses the principle of proportional control.

Drawing of a centrifugal "flyball" governor

It is most obviously seen on steam engines where it regulates the admission of steam into the cylinder(s). It is also found on internal combustion engines and variously fueled turbines.

The device shown is from a steam engine. It is connected to a throttle valve and to the prime mover (not shown). The action of the governor is dependent on centrifugal force. As the speed of the prime mover increases, the central spindle of the governor rotates at a faster rate and the two masses move outwards, and this motion is translated by the series of rods and arms to the throttle valve, reducing its aperture. The rate of steam entering the cylinder is thus reduced and the speed of the prime mover falls. If the speed of the prime mover falls, the reverse effect occurs and the throttle valve opens further.

James Watt designed his first governor in 1788 following a suggestion from his business partner Matthew Boulton. It was a conical pendulum governor and one of the final series of innovations Watt had employed for steam engines. James Watt never claimed the centrifugal governor to be an invention of his own. Centrifugal governors were used to regulate the distance and pressure between millstones in windmills since the 17th century. It is therefore a misunderstanding that James Watt is the inventor of this device^{[citation needed]}.

A giant statue of Watt's governor stands at Smethwick in the English West Midlands. It is known as the flyball governor.

Another kind of centrifugal governor consists of a pair of masses on a spindle inside a cylinder, the masses or the cylinder being coated with pads. This is used in a spring-loaded record player and a spring-loaded telephone dial to limit the speed.

The centrifugal governor is often used in the cognitive sciences as an example of a dynamic system, in which the representation of information cannot be clearly separated from the operations being applied to the representation. And because the governor is a servomechanism, its analysis in a dynamic system is far from trivial. James Clerk Maxwell wrote a famous paper "On governors"^[1], which is quite frequently considered a classical paper in feedback control theory, in 1868. Maxwell distinguishes moderators (a centrifugal brake) and governors which control motive power input. He considers devices by Watt, Professor James Thomson, Mr. Fleeming Jenkin, Sir William Thomson, M. Leon Foucault and Mr Carl Wilhelm Siemens (a liquid governor).

In a largely overlooked passage of his famous 1858 paper to the Linnean Society (which led Darwin to publish his monumental On the Origin of Species) Wallace says of the evolutionary principle:

The action of this principle is exactly like that of the centrifugal governor of the steam engine, which checks and corrects any irregularities almost before they become evident; and in like manner no unbalanced deficiency in the animal kingdom can ever reach any conspicuous magnitude, because it would make itself felt at the very first step, by rendering existence difficult and extinction almost sure soon to follow.^[2].

The cybernetician and anthropologist Gregory Bateson would observe in the 1970s that though seeing it only as an illustration, Wallace had "probably said the most powerful thing that’d been said in the 19^th Century".^[3] Bateson revisited the topic in his 1979 book Mind and Nature: A Necessary Unity, and other scholars have continued to explore the connection between natural selection and systems theory.^[4]

1. ^ J.C.Maxwell "On Governors" Proceedings of the Royal Society of London, Vol. 16, 1868, pp. 270-283
2. ^ Wallace, Alfred. the Tendency of Varieties to Depart Indefinitely From the Original Type
3. ^ Brand, Stewart. For God’s Sake, Margaret. CoEvolutionary Quarterly, June 1976. Retrieved on 2007-04-04.
4. ^ Smith, Charles H.. Wallace's Unfinished Business. Complexity (publisher Wiley Periodicals, Inc.) Volume 10, No 2, 2004. Retrieved on 2007-05-11.