Siphon

A siphon is a device used to move liquids through tubes.

In its most common form, a siphon uses a tube shaped like an inverted "U." This specific shape allows liquid to flow upward, above the surface of a reservoir. It then travels over the top and discharges at a level lower than the original surface. This process occurs without the use of a pump. Instead, the movement is powered by the fall of the liquid under the pull of gravity. Siphons are important tools in many different fields, from engineering to biology.

Scientists use two main theories to explain how a siphon moves liquid uphill. The traditional theory focuses on atmospheric pressure and gravity. In this view, gravity pulls the liquid down the longer exit side of the tube. This downward pull creates a zone of reduced hydrostatic pressure at the top of the siphon. Because the pressure is lower at the top, the atmospheric pressure at the entrance pushes the liquid up into that low-pressure zone. This is similar to how a barometer or a drinking straw works.

Another explanation is known as the cohesion tension theory. This theory suggests the liquid is pulled over the siphon in a way similar to a chain fountain.

You can imagine a chain hanging over a pulley. If one side of the chain is longer and heavier, gravity pulls it down. This action pulls the lighter side of the chain upward. In this model, the liquid's own cohesive tension—the way its molecules stick together—pulls it over the rise. However, this model has limits. For example, it does not account for the fact that the height of the reservoir surfaces, rather than just the weight of the liquid, determines the pressure balance.

Both theories may be correct depending on the environment. The atmospheric pressure theory cannot explain how siphons work in a vacuum. A vacuum is a space where there is no significant atmospheric pressure to push the liquid. In a vacuum, siphons rely on cohesion tension to function. Conversely, the cohesion tension theory struggles to explain gas siphons or siphons containing bubbles.

There is also the flying-droplet siphon. In this device, surface tension pulls the liquid into separate droplets inside a sealed chamber. This prevents the liquid from making contact, which means cohesion tension cannot be pulling the liquid up.

Humans have used siphons for thousands of years. Egyptian reliefs from 1500 BC depict siphons being used to extract liquids from large storage jars. In the 6th century BC, the Greeks used a device called the Justice cup of Pythagoras in Samos. By the 3rd century BC, Greek engineers in Pergamon were also using siphons. In the 9th century, the Banu Musa brothers in Baghdad invented a double-concentric siphon. They detailed this invention in their Book of Ingenious Devices. Later, in the 17th century, researchers studied siphons to understand suction pumps and the limits of vacuum pumps.

Siphons appear in many practical and surprising ways. You might see them used in homebrewing beer or in science demonstrations involving tropical fruit punch.

In large-scale engineering, siphon spillways are used to drain reservoirs.

Even in nature, siphoning principles are at work. In vascular plants, water is moved through parts called the xylem. This happens through a process called transpirational pull, which uses tension to move water. Whether it is a small tube in a lab or a massive system in a plant, siphons show how gravity and pressure work together to move the world's liquids.