Science Puzzle
How Does a Toilet Cistern Know When to Stop Filling?
Flush a toilet and the cistern empties, then refills by itself. It stops at exactly the right level every time, without overflowing. There's no electricity, no sensor and no computer.
How does the cistern know when to stop filling?
The Answer
The water level switches itself off. A float rides on the water and is joined by an arm to the inlet valve. As the water rises, the float lifts the arm, which gradually pushes the valve shut. When the level is right, the valve is closed.
After a flush, the water level drops, the float sinks with it and the arm swings down, opening the valve wide. Water pours in. As the cistern fills, the float rises and the flow gets slower and slower, until the valve closes completely at the set level. Bend the arm or move the float, and you change the level it fills to.
This is a simple example of feedback: the result of an action, the water level, is used to control the action itself, the flow of water. Because rising water reduces the flow, the system settles at one level, and engineers call that negative feedback. The same idea runs a central heating thermostat, which switches the boiler off as the room warms, and a car's cruise control. In 1788 James Watt used spinning weights to control the speed of his steam engines in just this way.
The principle: Negative feedback. A float measures the water level and closes the inlet valve as the level rises, so the result of the filling controls the filling itself.