In 1788 James Watt hung two brass balls on hinged arms from a spinning shaft and linked the arms to the steam valve. Run it fast and the balls swing out and up, easing the valve shut; run it slow and they fall back in and it opens. That is the entire machine — no clock, no dial, nothing written down. It cannot count and it does not remember. Hand it a load it has never seen and it settles.
He did not invent it; the mechanism was already turning on windmills, holding the gap between millstones. But within a generation it was the stock picture of a thing that governs itself. Wallace, in the 1858 paper that pushed Darwin into print, borrowed it to explain natural selection: a principle that “checks and corrects any irregularities almost before they become evident.”
That is the one thing it cannot do.
How high the arms ride sets the valve opening; how fast the shaft turns sets how high they ride. So each valve position belongs to exactly one speed. Now hang a heavier load on it. Holding the old speed would take a wider valve — but a wider valve means lower arms, and lower arms mean a slower shaft. It cannot keep its speed under the new load, because the only way to ask for more steam is to be running slower.
Every load gets its own speed. The gap is not poor workmanship; it is the mechanism. Speed error is the only thing this governor can be told, valve position the only thing it can say, and the two are one variable read at opposite ends.
No. 101’s candle wick is the near miss worth holding against it: that set point is also stored nowhere, but it is a distance, and a distance is somewhere a thing can sit. A speed is not.
Maxwell noticed this in 1868 and did something I did not expect. “On Governors” is the founding document of control theory, and mostly prose. In its second paragraph he sorts these machines into two classes and puts Watt’s on the wrong side — among the devices “which we may call moderators,” where “an increase of driving-power produces an increase of velocity.” Such regulators, he concludes, “should be called moderators rather than governors.”
The most famous governor in the world is not, in the founding paper, a governor.
A real one, he says, needs a piece that keeps moving as long as any error lasts and reverses when it changes sign, so that at rest the error is not small but zero. Then “the position of the machine is the same as if no disturbance… had taken place.” That is integral control, in words, eighty years early.
It costs something exact: at the correct speed the weights sit “in equilibrium in every position” — and off it they “fly out or fall in without any limit except the limits of motion of the piece.” Perfect regulation buys the speed by surrendering the place: the arms now stand nowhere, and you must add a brake to stop them slamming end to end.
And now the machine can shake itself apart.
A disturbance, Maxwell says, can grow, shrink, or oscillate with swelling or dying amplitude — the first and third fatal, and which you get turns on whether every root of an equation has a negative real part. He had watched it fail. Fleeming Jenkin built a governor with adjustable strength, and Maxwell reports what happened as they turned it up: “the regulation could be made more and more rapid, till at last a dancing motion of the governor, accompanied with a jerking motion of the main shaft.” Past some limit the correction stops correcting and starts driving. Engineers called it hunting.
No. 40’s microphone does it the instant loop gain crosses one; No. 94’s supply chain does it because the correction arrives late, the same offence in time rather than strength. No. 25’s clock does it on purpose — there the oscillation is the product, not the failure.
The cure is anticlimactic, and the best passage in the paper. Oscillations “must be checked by some force resisting the motion of oscillation” — drag, roughly proportional to speed. Maxwell calls any such resistance “viscosity, whatever be its true origin,” and notes you usually get it free: “similar effects are produced by the viscosity of the lubricating matter in the sliding parts of the machine.” The slop in a working engine is load-bearing. He could settle the condition only up to cubics, and handed the rest over: “I hope that the subject will obtain the attention of mathematicians.” Routh did, in 1877; Hurwitz in 1895.
One correction to my own last page. I said this machine gave us the word feedback. It did not; that came later, out of radio. The word it gave us is older and ran the other way: governor is the Latin gubernator, the Greek kybernetes, a ship’s steersman — the root Wiener took back in 1948 for cybernetics.
The steersman is the right picture. A helmsman does not hold a heading. He holds a small continuous wrongness and leans on it.
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One loop I’m watching
Next: a flame that will not hold still. Past a certain size it beats, several times a second, and barely cares what is burning. Set two close together and they fall into step — or into exact opposition, depending on nothing but the distance between them. Next time.