In 1788, the engineer James Watt added a device to his steam engines called a centrifugal governor — a pair of spinning weighted balls linked mechanically to the engine's throttle. It's often cited as one of the first widely used practical feedback control systems: a device that senses a machine's own output and automatically adjusts that machine's behaviour to correct it, with no person watching or adjusting anything by hand.
Speed up, and the machine throttles itself back
Watt's governor worked through simple mechanics. As the steam engine ran faster, the spinning weighted balls, driven by the engine's own motion, flew outward under centrifugal force. That outward motion was mechanically linked to the throttle valve controlling steam flow, so a rising engine speed automatically pulled the throttle partly closed, slowing the engine back down — and a falling speed let the balls drop inward, opening the throttle back up. The engine's own current output continuously adjusted the input controlling it, with no operator required to watch a gauge and turn a valve by hand.
Naming the loop: feedback, and why negative is the useful kind
This is the essential structure behind what's now called negative feedback control: a system measures its own output, compares it to a target, and feeds a correction back into its own input to close the gap — negative because the correction pushes back against the direction of the deviation, damping it out rather than amplifying it. The same basic loop, formalised mathematically in the twentieth century, now governs everything from a home thermostat to a cruise control system to the autopilot on a modern aircraft, all running some version of the same core idea Watt's spinning balls demonstrated mechanically over two centuries ago.
What we're still unsure about
A well-tuned feedback loop corrects deviations smoothly, but a badly tuned one can overcorrect, overshoot the target, and oscillate — sometimes worse than doing nothing at all. Precisely tuning a control system to respond quickly without overshooting, especially in complex modern systems with multiple interacting feedback loops, remains a genuinely difficult, actively researched engineering problem rather than a solved formula that applies uniformly across every application, from a simple thermostat to a rocket's guidance system.
This sits inside Control Systems, one of eight topics in Mechanical Engineering, one of four domains in Engineering, one of seventeen subjects the app can quiz you on.