siphons

A siphon is a tube that enables liquid to flow from a higher reservoir to a lower one without using a pump, by exploiting gravity and atmospheric pressure. In its common form, a U-shaped tube is placed with both ends submerged in the source liquid; the outlet end is kept below the source level, and the tube is primed so it is filled with liquid.

The flow begins when the liquid column is continuous. Gravity pulls liquid down the outlet limb, while atmospheric pressure acting on the surface of the source reservoir pushes liquid up the rising limb. This combination sustains a continuous column of liquid, provided the liquid does not vaporize and the column is not broken by air. The maximum practical height of a siphon is limited by atmospheric pressure, roughly equivalent to a 10-meter water column at sea level; beyond this height the column would cavitate and flow would stop.

A siphon continues until the source liquid level drops to or below the outlet level, or air enters the tube or the outlet is blocked. Stopping a siphon can be done by lifting the outlet above the source or sealing the tube.

Common applications include draining tanks, transferring liquids between containers, refilling fountains, and laboratory experiments. Siphons are also used in agriculture and aquaria. Variants include self-priming or pressure-regulated siphons, and devices with anti-siphon features to prevent backflow. The term siphon derives from the Greek siphon, meaning tube.