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siphon(sī`fən, –fŏn), tube or other enclosed conduit through which a liquid is lifted over an elevation and then emptied at a lower level. The movement of the liquid is driven primarily by the force of gravity. A siphon is typically shaped like an inverted J or U; to operate, it must discharge at a level lower than that of the liquid's surface on the intake side. The siphon must be filled before it will operate; suction is sometimes used initially to draw a liquid into a empty siphon.
a bent tube with legs of different lengths, through which a liquid flows from a vessel at a higher leveito a vessel at a lower level (see Figure 1). In order to start the operation, the siphon must first be filled with the liquid. The action of a siphon results from the fact that the pressure on the liquid volume filling the upper (hatched) section of the siphon applied from the direction of the upper reservoir, that is, from the left, is higher than that applied from the direction of the lower reservoir, that is, from the right. Thus, at the moment when the flow starts, the pressure on the left is equal to p0− γh1 and the pressure on the right is equal to p0 - γh2, where γ is the specific weight of the liquid, p0 is the pressure on the free surface of the liquid, and h2 > h1. In this manner, when the liquid is flowing
through the siphon, a pressure is established in the upper section that is lower than p0. The greater the difference in the heights h2 - h1 and the greater the liquid’s energy loss in overcoming the resistance of the tube, the greater the pressure drop in the upper section. This circumstance limits the difference in the heights of the liquid and, consequently, the operation of the siphon; when the pressure in the flow is below a certain limit, the column of liquid is disrupted. When cold water at atmospheric pressure is being transferred by a siphon, the maximum difference in the heights is usually no more than 6 to 7 m.