368 ROBERT G. GREEN 



The steel torsion wire (D) is tightly stretched between two 

 binding posts and carries the balance beam, one end of which 

 (5) swings over the face of the adjustable scale (F), the 

 opposite end carrying a silver cup (C) swoing on watch jewels. 

 The balance beam can be swung into the horizontal or zero 

 position by means of the bar (H). 



The adjustable scale consists of a series of arcs of the same 

 radius but the segments are of different lengths and each seg- 

 ment is divided into the same number of units by cross lines. 

 The scale face (F) is movable horizontally on the scale carrier 

 {E) in the visible slot and is fixed in any position by means of a 

 thumb screw behind. This mobiUty allows any one of the 

 various sized arcs to be moved directly under the end of the 

 pointer (B). The scale is constructed in relation to the size 

 of the torsion wire so that the units of the scale represent sur- 

 face tension in dynes per centimeter. 



The weight of a drop of hquid falling from the pipette (A) 

 into the cup (C) will be proportional to the surface tension of the 

 liquid according to the formula. 



where y = surface tension 

 w = weight of drop 

 K = constant of dropping pipette. 

 The force producing torsion in the wire will be the drop weight 

 times the acceleration of gravity and the amount of torsion pro- 

 duced will be proportional to this force. The degree of torsion 

 indicated by the pointer is therefore directly proportional to the 

 surface tension of the liquid dropping from the pipette. The 

 mobility of the scale makes it possible to di , ide any arc indicated 

 by the pointer into a desired number of units by moving under 

 the end of the pointer the proper sized scale arc and in this way 

 an arc of torsion that is proportional to a surface tension of sixty 

 dynes may be divided into sixty units and then each unit will 

 represent a djme of force. 



The surface tension balance is caUbrated by means of a stand- 

 ard liquid of high surface tension as follows : A drop of the liquid 

 of known surface tension is dropped from the pipette into the cup. 



