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When it gets warmer outside, the ball, representing room temperature, gets pushed to the right, toward higher readings. When it cools off, the ball is dragged back to the left. In neither case does the ball "push back"; it moves totally at the whim of the weather.

Providing negative feedback with a thermostat changes the lay of the land. Instead of the temperature roaming freely like a ball on a flat surface, now it's confined within a valley.

The flat floor of the valley represents temperatures between 60 and 80 degrees. Within this range the ball, representing the temperature, is free to follow the outside temperature as before. Once it reaches the point where the thermostat starts heating or cooling the room, however, it encounters a steep slope. As long as the heating or cooling capacity exceeds the rate heat can leak through the walls from outdoors, the temperature will remain confined to the desired range. This viewpoint makes the stability conferred by negative feedback self-evident.

Negative feedback is at the heart of every stable, self-regulating system. If a company raises prices too high, people stop buying and soon the company cuts the price to increase sales. When there's plenty of rabbits, the wolves eat well, multiply, and before long more rabbits get eaten and they aren't so abundant. When your car starts veering into the left lane, you turn the wheel to the right. When you're feeling tired, you get more sleep. Negative feedback is ubiquitous, yet its profound consequences are often overlooked for this very reason. To understand them better, consider what happens when feedback goes bad.

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