Siphon
A tube that moves liquid uphill using gravity alone.
A siphon, also spelled syphon, comes from the Ancient Greek word for "pipe" or "tube." Broadly, it describes any device that moves liquids through tubes. More specifically, it usually means an inverted U-shaped tube that lets liquid flow upward, above the surface of a reservoir, without a pump. Gravity powers this flow as the liquid falls down the tube and exits at a level lower than the reservoir's surface.
Two main theories explain how siphons make liquid go uphill against gravity, powered only by gravity itself. The older theory, held for centuries, says that gravity pulling liquid down on the exit side creates lower pressure at the top of the siphon. Atmospheric pressure then pushes liquid from the upper reservoir up into that low-pressure zone, similar to a drinking straw or barometer. However, siphons have been shown to work in a vacuum and at heights greater than the barometric limit of the liquid. This led to the cohesion tension theory, where the liquid is pulled over the top like a chain fountain. Neither theory is necessarily the only correct one; both may apply under different ambient pressure conditions. The atmospheric pressure theory cannot explain siphons in a vacuum, where there is no significant air pressure. The cohesion tension theory cannot explain siphons using CO2 gas, siphons that work despite bubbles, or the flying droplet siphon, where gases don't pull strongly and liquids not in contact can't exert cohesive tension. All modern published theories recognize Bernoulli's equation as a reasonable approximation for idealized, friction-free siphon operation.
**History** Egyptian reliefs from around 1500 BC show siphons used to take liquids from large storage jars. Greek physical evidence includes the Justice cup of Pythagoras on Samos from the 6th century BC and use by Greek engineers at Pergamon in the 3rd century BC. Hero of Alexandria wrote extensively about siphons in his work *Pneumatica*. In 9th-century Baghdad, the Banu Musa brothers invented a double-concentric siphon, described in their *Book of Ingenious Devices*; an edition edited by Hill includes an analysis of this device. Siphons were studied further in the 17th century alongside suction pumps and early vacuum pumps, especially to understand the maximum height of pumps and siphons and the apparent vacuum at the top of early barometers. Galileo Galilei initially explained t
- first_known_depiction
- 1500 BC
- ancient_culture
- Egyptian, Greek, Islamic
- key_historical_figure
- Hero of Alexandria
- field
- Physics, Hydraulics
- known_for
- Inverted U-shaped tube that moves liquid uphill without a pump
Lore & Background
Egyptian reliefs from 1500 BC depict siphons used to extract liquids from large storage jars. Physical evidence for the use of siphons by Greeks includes the Justice cup of Pythagoras in Samos in the 6th century BC and usage by Greek engineers in the 3rd century BC at Pergamon. Hero of Alexandria wrote extensively about siphons in the treatise Pneumatica. The Banu Musa brothers of 9th-century Baghdad invented a double-concentric siphon, which they described in their Book of Ingenious Devices.
Reader's Guide
Siphons are significant because they demonstrate how liquids can flow uphill against gravity without a pump, powered solely by gravity. Two leading theories explain their operation: the atmospheric pressure with gravity theory, which holds that reduced pressure at the top of the siphon allows atmospheric pressure to push liquid upward; and the cohesion tension theory, which suggests the liquid is pulled over the siphon like a chain fountain. Both theories may be correct in different circumstances, as siphons can operate in a vacuum (where atmospheric pressure is absent) and also with gases or bubbles (where cohesion tension fails). Bernoulli's equation is recognized as a decent approximation to idealized, friction-free siphon operation. Historical study of siphons in the 17th century, in the context of suction pumps and vacuum pumps, helped clarify the nature of atmospheric pressure and vacuum, disproving earlier theories such as horror vacui.
Did You Know?
- Egyptian reliefs from 1500 BC depict siphons used to extract liquids from large storage jars.
- The Banu Musa brothers of 9th-century Baghdad invented a double-concentric siphon.
- Siphons can operate in a vacuum, where there is no significant atmospheric pressure.
- The flying-droplet siphon works continuously without liquid tensile strength pulling the liquid up.
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