Siphoning Nonsense

The other day I was watching Rachel Maddow’s show on MSNBC when on came their geek spot segment. The story this time? “The dictionary definition of the word ‘siphon’ has been wrong for nearly a century.”

“What the –?” I said to myself.

I was then informed that according to some college lecturer in Australia the Oxford English Dictionary was wrong in attributing the action of a siphon to atmospheric pressure. “It’s just gravity,” they said.

Wondering whether I could believe my ears that all the college-educated journalists on MSNBC could be so ignorant of middle-school physics, I found that the story was being sourced to the blog of the Guardian, the British daily. Indeed, there there is a piece by James Kingsland saying just that. He goes on to say that gravity pushes the liquid. I recall Newton telling us that gravity is an attractive force – when did it start pushing?

Later on Kingsland decides that the water is pulled up the shorter pipe by “the continuous chain of cohesive bonds between the liquid molecules.” So water is a sticky string! The lecturer he cites on this is Stephen Hughes at Queensland University of Technology in Brisbane.

The Daily Telegraph had the same story, which doesn’t surprise me. The Telegraph has a long history of incompetent science reporters, including one who abused me personally in print after I pointed out one of his blunders. But the Guardian is usually not quite as clueless. Admittedly this is the blog rather the print edition but still, do they not care who they let on to their turf?

The problem of ignorance of science and technology among highly-paid journalists in the mainstream media was ever thus. Many in the media think it doesn’t matter – after all, the only people who will notice what’s wrong are geeks. But hold on folks – science and technology are all around us. They happen to be very important, and they impact everyone’s lives. In a democracy we cannot afford public ignorance of a major part of public policy.

It isn’t just that the journalists get details wrong – often there are elephant-in-the-room questions that occur to any knowledgeable person and that have to be answered in order for the story to make any sense at all. And yet the editors don’t seem to mind that their pages and airwaves are being filled with drivel, whether it’s energy policy, the environment, industrial development or the cure for cancer.

Looking back at the story, it didn’t occur to Kingsland to ask, “Could a siphon work in a vacuum chamber? Could you have a water siphon taller than 32 feet?” A little bit of knowledge would have told him that at the top of the short tube, the pressure in the liquid is atmospheric pressure minus the weight of the liquid. At the top of the long tube the pressure is likewise atmospheric pressure minus the weight of the liquid. Since there is more liquid on the long side, the pressure at the top on the long side is less than on the short side. Consequently the liquid flows from the higher pressure to the lower and down the long tube.

So Kingsland clearly got his job as a subscriber to the old journalist maxim that you should never let the facts get in the way of a good story. How the lecturer got his job is another question.

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1 thought on “Siphoning Nonsense”

  1. Re: Siphoning Nonsense


    It appears that the primary thrust of this article is to chastise “highly-paid journalists in the mainstream media” for their “ignorance of science and technology”. However, the definitions of siphon I have seen that claim “that atmospheric pressure forces the liquid through the tube”, or similar phrasing is just as incomplete as the assertion that “It’s just gravity”.

    The truth, as is often the case with science, is more subtle, and involved than either. Basically, the action of the siphon depends on both gravity and atmospheric pressure (as well as cohesion within liquids: one of the distinguishing features between liquid vs. gaseous fluids).

    As you say, the siphon will not work within a vacuum chamber, with “normal” liquids. (Of course, such “normal” liquids will simply vaporize with a sufficiently hard vacuum.) However, regardless of the air pressure within a chamber, the siphon will also not work without gravity.

    For an essentially incompressible liquid (like water), drawing the liquid through a tube will work due to the cohesiveness of the liquid, so long as the local pressure within the liquid is sufficiently large that the liquid does not transition to the vapor phase (vaporize), since this vapor phase does not have cohesive characteristics anywhere near that of the liquid phase.

    So, in a sense, the only “function” of the atmospheric (external) pressure, on a siphon, is to keep the local pressure of the liquid column, everywhere within the column (at the top is, of course, the most critical point) from dropping below the vaporization pressure for the liquid, at its local temperature. Therefore, the critical height for a siphon is dependent upon the liquid used, its temperature, and the external pressure.

    On the other hand, the actual action of the siphon itself (provided the conditions are met to keep the liquid from vaporizing) is due to gravity (the imbalance of the weight of the fluid on the two legs of the siphon, in a way very similar to a rope hung over a frictionless pulley).


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