ELECTROSTATICS 



21 



If a charge of q units per centimeter length is given to the 

 inner cylinder, lines of dielectric flux pass out radially and pro- 

 duce, at a distance r cm. from the axis of the cylinder, a flux 

 density 



3) = ~= = lines per square centimeter. 





FIG. 16. Capacity of concentric cylinders. 



If the dielectric constant of the material is Kj the electro- 

 static force at the point is 



^ = f = |? dynes, 



and the difference of potential between the two cylinders is 



r 2 q dr 2q, a , orvv 



E= J b F7 = Z log 6 ergs (39) 



The capacity per centimeter length of the condenser is 



E 25, a 



electrostatic units. . (40) 



This is the case of a single conductor cable with a lead sheath, 

 and the capacity is usually expressed in farads per mile; it is 

 K 2.54 X 12 X 5280 



K 



9 X 10 11 



-. 2.54 X 12 X 5280 



