Electricity of Drops. 355 



sign. Other considerations show, however, that this differ- 

 ence cannot amount to anything like a volt, for otherwise the 

 difference between the surface-tension of distilled water and a 

 weak electrical neutral solution of, say, sodium chloride would 



Fig. 3. 



oc 



be far greater than it is. Let V be the difference of potential 

 between the drop and the air, S the area of the surface of the 

 drop, t the distance between the two coatings of electricity ; 

 then the two coatings may be regarded as the charged plates 

 of the condenser, whose potential energy is 



1 SY^. 



2 \-trt ' 



or, if Q is the charge on either coating, 



c Q 2 



The existence of this double coating will thus diminish the 

 apparent surface-tension by 



Q 2 



27T£-q2"> 



