Posted on 05/11/2010 9:03:35 AM PDT by Gomez
An Australian physics prof has discovered a 99-year-old error in the Oxford English Dictionary - repeated in most dictionaries worldwide - and is having it corrected.
The error is in the definition of the noun "siphon", a tube used to draw fluid from a higher location to a lower one - as when emptying a vehicle fuel tank, an aquarium or other vessel difficult to empty by other means.
Liquid is, of course, drawn up the shorter limb of the siphon by the weight of that in the longer downward one: thus the operating force is gravity. However most dictionaries follow the OED in stating that atmospheric pressure drives the process.*
Dr Stephen Hughes of Queensland University of Technology noted the error after visiting a massive siphon project in South Australia which was being used to transfer gigalitres of water into a depleted lake.
On returning, the prof decided to write an article about the siphon for use by school science teachers, and discovered to his dismay that most dictionaries described the process wrongly.
"An extensive check of online and offline dictionaries did not reveal a single dictionary that correctly referred to gravity being the operative force in a siphon," grumbled the physicist.
The OED currently says:
A pipe or tube of glass, metal or other material, bent so that one leg is longer than the other, and used for drawing off liquids by means of atmospheric pressure, which forces the liquid up the shorter leg and over the bend in the pipe.
"The OED entry for siphon dates from 1911 and was written by editors who were not scientists," explained Margot Charlton of the Dictionary's staff. Amazingly, it seems that in 99 years nobody had ever queried the definition.
The next edition of the OED will be corrected.
According to Hughes some encyclopaedias - though not the Encyclopaedia Britannica - repeat the error. The doc has written a paper with more detail on siphons which the interested can read here.
*This may be true during the process of starting the siphon off, which is usually done by creating a temporary suction on the outflow end of the pipe so as to draw fluid up and over the hump. This works by the action of atmospheric pressure on the surface in the to-be-emptied vessel: but once the siphon is flowing this force is countered by atmospheric pressure at the other end of the pipe.
All pythons can siphon.
--
/ \
/ /\ \
/ / \ \
/#/ \ \
/#/ \ \
| /#/ | \ \
|###/#/###| \ \
|#########| \#\
|#########| \#\
+---------+ \#\
|###\#\###|
|#########|
|#########|
+---------+
This is what will happen to a siphon in a low pressure environment.
No, sucking it requires atmosphere.
Remember you need to suck on the siphon to start it (ie raise the height of the column of liquid)...once the liquid passes the high spot, it starts falling down the other side, which continues the sucking by itself.
Sucking is not required to start the siphon. Several posters already described methods of starting a siphon without sucking on the tube: Immerse the entire tube in the reservoir so the tube is filled with liquid. Stopper one end with your finger so the tube remains filled as you lift it over the edge of the container. Release the tube when the end is lower than the level in the reservoir and viola! It flows.
I live in the desert, and farmers are always using the method I described to get siphons flowing to irrigate their fields from a canal. They don't suck the water into the tubes to start them.
That is interesting to ponder....
However, this Ozscience wannabe boffin is operating in the antipodal region and due to the Coriolis Effect, his mind has been weakened by bath water swirling the wrong way since childhood,crikey, and stone the crows.
Also, being raised on wichitee grubs and chased by salties has been shown to stunt the growth of cerebral cell structures. Treat with Foster's frosty amber fluid (comes in tubes) and bed rest.
The water gets to the top of the trees because of capillary action. An entirely different phenomena.
If you have sufficient stickiness with the surface of the tube, the pressure at the top does not go to zero. You could have the same pressure at the top as the bottom and the liquid will not go down. Try it with a really skinny tube, say 1/100 inch, that is open at the top. The column will stay in place by viscosity alone!
(Try it with supercooled liquid helium and because of its infinite viscosity, it will siphon itself out of a glass with no tube at all!)
:^)
Best explanation thus far....especially the Fosters part.
Not entirely different.
How about supercooled liquid helium? hmmmmm?
With three fish tanks in the house, you bet our python siphons!
Use a skinnier, stickier tube.
I haven’t sen a tree that consumes supercooled He, but it could happen....The thing with trees is H2O & tall trees, is surface tension. I don’t know if it’s a water thing or a liquid thing...
So does that mean I can't have a water well more than 33 feet deep?
OK, I’ve thought about your drawing a bit. The pressure is required to keep the tube filled with water, but it’s not responsible for the flow of the siphon.
When my granddaughter was 3, I took her outside and put potato chip crumbs on the walk.
100's of ants paraded down the walk. You'll never find chips in her bookbag.
I didn’t say that...I was quoting someone. That’s what I use italics for....
When I lived in a part of the country that required a water well, we had the pump at the bottom of the well to push the water up. I don’t think you can suck the water up that distance, but a well is a special case because the aquifer has different forces at work on it besides atmospheric pressure. There are places in the country where that aquifer is under pressure, and you don’t even need a pump to get the water.
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