RAPID MEASUREMENT OF SURFACE TENSION 369 



The pointer then swings into some position over the face of the 

 scale. The scale face is then moved horizontally until the reading 

 off the end of the pointer is the same as the surface tension of the 

 liquid used in calibrating. The balance is then ready for use 

 and if a liquid is now used which has one half the surface tension, 

 the weight of the drop will be one-half, the torsion produced in 

 the wire will be one-half and the reading on the scale will be 

 one-half the previous reading. 



The sensitiveness of the apparatus varies with the mechanical 

 construction depending upon the ratio of the mass of the balance 

 beam to the torsion constant of the torsion wire. The number 

 of drops of hquid used in determinations can vary from a single 

 drop to many if a torsion wire of the proper size be chosen. Five 

 drops have been found very satisfactory in our work. 



Evaporation is the greatest source of error with this apparatus. 

 It cannot be used for very volatile Uquids. However, if cali- 

 brated with water and used for watery solutions, the error from 

 evaporation is rendered neglible in that under like experimental 

 conditions, the same evaporation takes place in calibration that 

 occurs in actual measurements. This has proven out in that 

 repeated runs of watery solutions give identical results to the 

 limit of readability of the scale. The temperature variation may 

 be rendered negligible by controlling temperature or proper cor- 

 rections may be made. The accuracy of the surface tension 

 balance now in use is plus or minus one-tenth dyne. 



The pipette is the only part of the apparatus to be cleaned as 

 the cup need only be dried and the pointer reset to zero. The 

 pipette may be cleaned very rapidly if suction and compressed 

 air are available. Since the time consuming weighings are 

 eliminated the measurements may be taken with great rapidity. 

 With this apparatus the surface tension of thirty solutions were 

 measured in one hour. 



SUMMARY 



An apparatus termed a surface tension balance is described 

 for the rapid measurement of surface tension by the drop-weight 



