246 Stevens — Method of Measuring Surf ace Tension. 



the pull is then steadily applied to the center of inertia of the 

 system. 



The magnetic force may be calibrated at leisure by use of a 

 balance or it may be calculated from the constants of the coil. 

 This in proper units must be added to the weight of the 

 system. 



In some experiments already made by this method a circu- 

 lar disk of thin ferrotype plate was used and a rather heavy 

 sinker of soft iron fastened to its center. The variation of 

 surface tension with temperature in case of distilled water, 

 the effect of adding a drop of oil, the surface tension of a 

 soap solution and the results with disks of different areas have 

 been tested. 



In order to test the law of inverse squares a hook gauge 

 was employed so that the height of the water surface, and 

 hence the floating disk, could be adjusted very closely. By 

 means of a scale fastened to the side of the vessel containing 

 the liquid, the weight of the surface could be changed by suc- 

 cessive centimeters. If we assume that the force required to 

 break the tension is constant, and if we call the distance 

 between the center of attraction of the coil and the magnetic 

 center of the system attracted, when they are nearest together, 

 x • then x+1, x-\-2, etc., will represent other distances used. 

 Let c', c", etc., stand for currents corresponding to these dis- 

 tances and we have 



c 



etc. 



x x + 1 x + 2 



Substituting values and solving we have for a given experi- 

 ment x = 6 cm . 



Applying the values to other measurements we have 1*88, 

 1*81, and 1*95 as ratio numbers expressing the value of the 

 same force, namely that required to break through the surface 

 tension. 



Very little work has been done with this apparatus and this 

 paper is designed to describe the method rather than announce 

 results. 



Orono, Maine. 



