Why is Cecil pulling his punches in his otherwise admirable article “How does a siphon work” on the question of how a siphon works? Cecil is absolutely right in saying that air pressure explains the operation of a siphon, but he lets the Encyclopedia Britannica off too lightly.
According to Cecil, the encyclopedia says: The action of a siphon depends upon the influence of gravity (not, as sometimes thought, on the difference in atmospheric pressure —a siphon will work in a vacuum) and upon the cohesive forces that prevent the columns of liquid in the legs of the siphon from breaking under their own weight.”
The encyclopedia’s statement has so, so much wrong with it, as I shall demonstrate.
No one is saying that a siphon doesn’t depend on the influence of gravity and it isn’t “sometimes thought” that the action of a siphon depends on the difference in atmospheric pressure. Everyone accepts that without gravity, a siphon cannot operate. As for the difference in atmospheric pressure, that will hinder the operation of a siphon, because atmospheric pressure increases as you go lower, which everyone knows. In any case the difference in atmospheric pressure is of such a small magnitude as to be negligible when water is being siphoned. It’s a straw argument. What really is sometimes thought, and correctly, is that an ordinary siphon depends on the presence of atmospheric pressure. By “ordinary siphon” I mean a siphon using water found in the environment on earth, rather than some other fluid, such extremely pure water found only in a laboratory.
There’s no way an ordinary siphon depends on cohesive forces, because for cohesive forces to play a role the water has to be under tension, and the water in it is never under tension. It’s under pressure. Every part of the water in the siphon is under pressure to some degree. If it were not, it wouldn’t be able to exist in a liquid form. It would start boiling at room temperature (so much for a siphon working in a vacuum). Air pressure is what keeps the water under pressure in the tube, keeping it in the tube and in one piece, keeping it liquid, and pushing it through the tube towards the lower end.
I don’t think the encyclopedia can explain the observed bubbles that, if not too large, can be carried through a fast-moving siphon. In many cases the bubbles are large enough to completely separate the water, and yet the siphon still works. So much for the importance of the water in the legs of the siphon from not breaking under their own weight.
The encyclopedia ignores the fact that a dense enough gas can be siphoned, though not in a vacuum. Clearly there is no cohesiveness of a gas. For example cold air is dense enough to be siphoned in air at room temperature.
I have read that extremely pure water, unlike ordinary water can exist even under negative pressure, that is to say, the water is pulling on the containing walls, and this negative pressure can be several times atmospheric pressure in magnitude. It’s hard to believe, but it’s also hard to believe someone just made it up, so I guess there’s some truth to it, and ultrapure water can really be under tension. But this doesn’t mean the water is under tension in an ordinary siphon. For that to happen you’d need to be siphoning ultrapure water, and you’d need to have a height of more than about 34 feet.