Most people are familiar with the use of a siphon for removing water from fish aquaria or other receptacles which cannot otherwise be emptied conveniently. The siphon is a bent tube made of glass, rubber, or plastic tubing with its short arm dipping into the tank of liquid and its longer arm outside (Fig. 11.9). To start the siphon it must first be filled with liquid. After this the liquid will continue to run out so long as the end E is below the level of the liquid in the tank. At one time it was generally accepted that a siphon worked by atmospheric pressure. But there is now strong evidence to support the view that cohesion between the liquid molecules plays an important part.
The pressures at A and D in the two limbs of the tube are both equal to atmospheric pressure, since they are at the same horizontal level as the surface of the liquid in the tank. The pressure at the outlet E is equal to atmospheric pressure plus the pressure hpg due to the column DE. The excess pressure, hpg, therefore causes the liquid to flow out of the tube at E. Also, since the liquid has to rise a distance AB up the tube, it follows that the siphon will fail to work if AB is greater than the barometric height in terms of the liquid being used.
Experiment shows that siphons can be made to work in a vacuum and that, in certain cases, the flow will continue even if AB is somewhat greater than the appropriate barometric height. The presence of atmospheric pressure, therefore, does not appear to be essential. It would seem that the flow of liquid occurs owing to the greater force of gravity on the column CE, which pulls the shorter column AB through the cohesion (attractive force) between the liquid molecules. The action can be likened to that of a chain passing over a freely running pulley which will run off in the direction of the longer
ide (Fig. 11.10).
Only pure liquids will siphon satisfactorily in a vacuum. If dissolved gases are present the cohesive force between the molecules is greatly lowered and bubbles readily form under reduced pressure. Hence, atmospheric pressure is a necessary condition in the case of impure liquids as it compresses the liquids in the tubes and prevents breakage of the liquid columns through the formation of bubbles from dissolved gases. A more detailed discussion of the action of the siphon will be found in an article written by M. C. Nokes in the School Science Review, Vol. 29, p. 233. Anyone who is really interested in the subject may read Hydraulic design of syphons”; A. G. Kelly B.Eng. (Proc. Inst. Mech. Engrs., 1965-6) vol. 180, p. 981. They may come to the conclusion that the last word on the siphon has yet to be written.