104 



which differs only by a constcait factor from the electro- 

 static energy 



of the electrical problem. The chan£_,e in the latter v;hen 

 the bubble is brought from infinity to a distance h from 

 the plane is equal to the work done in this operation. If 

 h is a few tines larger than the radius a of the bubble, the 

 field E acting on the bubble, due to its image, when it is 

 a distance x from the plane, can be taken as — a /4x , with 

 the upper sign in case (a), the loT;er in capo (b). The 

 work done, thei-'of ore, is 



cbc z -^ ~^^ (4) 



— CO 



