STALAGMOMETER 



183 



then applied in which the weight of the drop W is proportional to the 

 surface energy and radius of the tube. 



W = 2-irrT 



It is assumed that the tension of the liquid acts vertically around the 

 rim of the tube from which the drop is suspended and subsequently 

 necks off. 



It can be shown theoretically and verified experimentally that drops 

 necking off from a vertical tube are always smaller than 2-irrT, often by 



Ky 



o 



Fig. V-3. Progressive stages in the formation of a drop of water separating 

 itself from a clean polished surface at the end of a glass capillary tube. Freehand 

 sketch. 



more than 40 per cent. High-speed photographs by Guye and Perrot 

 [1903] show that the suspended drop becomes unstable before it leaves 

 the end of the tube. Then a constricted portion develops, as seen in 

 Fig. V-3, which narrows and subsequently allows the drop to form after 

 breaking off near the end. The stretched column between the lower 

 drop and the tube then separates itself from the tube and forms an 

 additional small drop; finally, the remainder of the column retracts, 

 allowing some of the liquid to remain suspended from the end of the tube. 



In any drop-weight method both the large and small drops must be 

 counted together as " one drop." 



Harkins and Brown [1919] showed by careful measurements that the 

 weight of a drop is conditioned by the radius of the tube from which the 

 drop falls and is also a function of the inverse cube root of V, the volume 

 of the drop. The more exact relation between these quantities is 



or more simply 



W = 2-rrrTf 



T= m F 



\v 1/3 ) 



