THE MEASUREMENT OF SURFACE 



TENSION 



By WILLIAM N. RAE, M.A. (Cantab.), and JOSEPH REILLY. M.A., 



D.Sc. (N.U.I.). F.R.C.Sc.I. 



Of the many methods which have been devised for the measure- 

 ment of surface tension, two only have been used to any extent 

 in physico-chemical work. This is largely due to the fact that 

 most of the methods which are otherwise satisfactory do not 

 lend themselves to accurate temperature control {e.g. the method 

 of the wave length of ripples), and since the temperature effect 

 is of the order of 0*2 per cent, per degree, accurate temperature 

 control is necessary. The two which have been most largely 

 used are the capillary rise method and the falling drop method. 

 The former depends upon the fact that, when a capillary tube is 

 dipped into a liquid, the liquid rises to a height {h) in the tube 

 given by the relation 



2 T cos CO = rg{p — a) {h —x ) 



where T is the surface tension in dynes per sq. cm. 



0) is the angle of contact of the liquid with the walls of 



the tube. 

 r is the radius of the capillary tube in cm. 

 g is the acceleration due to gravity in cm, per sec. per 



second. 

 p is the density of the liquid, and a that of the vapour 



or gas with which it is in contact. 

 X IS 2i correction factor for the liquid above the 



level of the bottom of the meniscus of. the liquid 



in the tube. 



The data given in Landolt-Bornstein's tables show that the 

 values obtained by different experimenters for the surface 

 tension of water at 20° C. by the capillary rise method vary 

 between 70-6 and 727, while the values by all methods vary 

 between 70-6 and 78. Thus, although a single experimenter gets 



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