Siphon, a bent tube used to move a liquid over an obstruction to a lower level without pumping. A siphon is most commonly used to remove a liquid from its container. The siphon tube is bent over the edge of the container, one end in the liquid and the other outside end at a lower level than the surface of the liquid in the container. If the tube is once filled, a flow of liquid from the container through the tube will be set up. Several methods can be used to fill the tube. A small pump may be used. (The pump is no longer necessary once the flow has begun.) Water or any other harmless liquid may easily be siphoned through a small, short tube by sucking it through the tube with the mouth, as through a drinking straw, until the flow is started. The tube may also be filled by submerging it completely and then covering both ends while it is placed in position.
Siphons are sometimes used in irrigation to lift water from the irrigation canal, over a dike, and into a field. Wine may be siphoned from the top of large winemaking vats without disturbing the sediments on the bottom. Aqueducts sometimes act as siphons in carrying water over elevations.
Siphons operate by atmospheric pressure. The container from which the liquid is siphoned must therefore be open to the air. When the tube is filled, the liquid will run out of the lower end. (The greater weight of the liquid in the arm outside the container determines the direction of flow of the liquid.) As the liquid starts to flow, the fluid pressure at the top of the tube is lowered. A liquid always flows from an area under higher pressure to an area of lower pressure. The liquid in the container (under atmospheric pressure) flows up into the tube (an area of lowered pressure). This liquid in turn will flow out the outside end of the tube, again lowering the pressure at the top of the tube.
Once the flow has begun, it will continue if undisturbed as long as the inside end of the tube remains below the surface of the liquid. The flow can be cut off by raising the outside end of the tube above the level of the surface of the liquid in the container.
One limit to the use of siphons is imposed by the height to which atmospheric pressure can lift a given liquid. At sea level, atmospheric pressure can raise water to a height of about 30 feet (9 m). At higher altitudes the pressure is less, as is the height to which the water can be raised. Liquids heavier than water cannot be raised as high as water. Thus at sea level, mercury can only be raised about 30 inches (760 mm).