522 APPARATUS FOR MEASURING SURFACE TENSION 



Some authors give 82 for the first method, others, 80, and for the 

 third, some give 68. 

 Temperature plays an important part. It may be represented by 



the simple formula: 



A = Ao (l—at) 



The cleanliness of the apparatus containing the liquids and of the 

 measuring devices is extremely important, for a minute trace of greasy 

 substance spoils the results. 



The importance of the action of surface tension in biological phe- 

 nomena is well known, but all the techniques of measurement are either 

 comphcated (static methods: capillary undulations, drop rebounding, 

 capillary jets, rebounding jets), or long (drop method), and it is de- 

 sirable to have a simple apparatus by means of which the surface 

 tension, and especially the variation of surface tension of a given 

 liquid, could be readily measured, with sufficient accuracy. For this 

 reason the apparatus to be described has been designed (Fig. 1). 

 There is no new principle in it; it is based upon adherence of a ring, 

 or of any other design, to the liquid (Weinberg). It is simply a 

 torsion balance, but instead of measuring the tension by means of 

 weights (which is time-consuming, and makes two readings neces- 

 sary), the torsion of the wire is used to counteract the tension of the 

 liquid film and to break it. A single reading on a dial indicating the 

 degree of torsion of the wire gives a figure, which, if the apparatus 

 has been previously standardized with water, gives the surface ten- 

 sion of the liquid by a simple proportion. According to the fact that 

 the torsion of the wire for water, which has the highest surface ten- 

 sion, is only 72°, we can assume that, within these limits, the strain of 

 the wire is proportional to the angle of torsion, so that no table of 

 correction is needed (see Fig. 2). Of course, the apparatus may be 

 used for standard measurements also, by simply forcing the lever 

 to come back to its former position by means of weights after the 

 tearing of the membrane has taken place. By using the comparison 

 method, the figures are reliable and constant, and the time neces- 

 sary to make one measurement does not exceed 15 to 30 seconds. 

 A very small quantity of the liquid is required, — about 1 cc. in a 

 watch-glass, — but the apparatus could be fixed in such a way that 

 it would need only 0.5 cc. 



