Electrostatic Capacity of a Conductor. 1 1 7 



Assuming the lines of force to be parallel, the section of 

 the tube of force is evidently independent of r or 



or 



From this example it is seen that the specific capacity 

 of a dielectric (as defined above, namely, the capacity of a 



unit cube) is equal to — , and that the capacity of a portion 



of a dielectric is got by multiplying the specific capacity by 

 the cross section, and dividing by the length of the dielectric, 

 assuming that the cross section of the portion of the 

 dielectric considered remains uniform. If it does not, we 

 must have recourse to a process of integration, as shown 

 above. 



Ex. 3. — Coaxal cylinders, of length /, and radii Rj and 

 R2. 



In this case, the lines of force are evidently radii between 

 the two cylinders. Con- 

 sidering a tube of force 

 formed by two planes pass- 

 ing through the axis, we 

 have 



J 



R.. 



d^ ~ Id^^^^^ 



Ri 



The limits of are evi- 

 dently o and 2ir. 



Fig:- 3- 



