ON THE SIPHON 



By William Dcane 



The writer wishes to call attention to an error that has crept 

 into the text-books on general physics, written for high-school and 

 university classes. Most of the books either state explicitly that a 

 siphon will not work if the shorter of its two legs is longer than the 

 column of liquid that would be supported by the air pressure, or else 

 give explanations of the siphon, from which this follows as a legiti- 

 mate conclusion. As a matter of fact, a siphon can be made to work 

 and draw the liquid to a height considerably greater than that repre- 

 senting atmospheric pressure. 



The writer usually illustrates this fact in his lectures by means 

 of the following simple experiment: Let ABC in the figure be a 

 glass siphon tube, both legs of which are 10 cm. or 15 cm. longer 

 than the barometric column. The bore of the tube should be small 

 (about j\ sq. mm.) to work well. Let one of the legs, BC, dip 

 down into a larger tube, CD, partly filled to D with mercury. Fill 

 ABC with mercury, and start the siphon drawing mercury from C 

 over to A in the usual way. In order to start the siphon the vertical 

 height of B above the surface D of the mercury should be less than 

 the length of a mercury barometer column, but as the flow con- 

 tinues, the mercury surface descends and keeps on descending until 

 its vertical distance below C is considerably greater than this length. 



To make this experiment work suificiently well for demonstra- 

 tion purposes, excessive care in purifying the mercury and cleaning 

 the glass is not necessary. Boiling the mercury in the actual tubes 

 used, for instance, is superflous. With ordinary redistilled commer- 

 cial mercury and tubes cleaned with alcohol the writer has made the 

 siphon work to a height of seventy centimetres. As the altitude of the 

 University laboratory, where the experiment was performed, is a 



