The first machine that ruled itself. Two heavy balls spin on arms off the engine’s shaft; spin faster and they fly outward, and rising, they close the steam valve that feeds the engine — so speeding up throttles itself down. A loop with no brain, holding a steady speed. Slide the speed and watch the balls set their own limit.
At steady spin each ball sits where gravity and centrifugal force balance: cosθ = g/(ω²L), so faster ω lifts the balls (θ grows) until, past √(g/L), they swing out. That rise mechanically closes the throttle, cutting the power that drives ω — negative feedback that pins the speed with no sensor or computer. A fail-loud self-check throws unless θ increases monotonically with ω AND the equilibrium ω² = g/(L·cosθ) holds exactly at the computed angle — the 1788 loop Maxwell later analysed to found control theory.
A static-equilibrium model — the real governor has mass, damping, and can HUNT (oscillate) if tuned badly, which is precisely what Maxwell’s 1868 ‘On Governors’ studied. The cosθ = g/(ω²L) balance and the feedback direction are exact.