158 



ADVANCED ELECTRICITY AND MAGNETISM. 



does not contain the radius of the outside cylinder so that this 

 radius may be as large as you please. 



95. Graded cable insulation. Equation (i) of Art. 94 shows 

 how the electrical stress e is distributed throughout an ordinary 

 electric cable which consists of a cylindrical wire core, a covering 

 of rubber or other insulating material and a lead sheath. If k 

 is the same throughout the insulating material, then e is much 

 larger near the central wire where r is small, than near the sheath 

 where r is large; but if it were possible to apply the insulating 

 material in layers with the value of k decreasing as r increases 

 so as to keep the product kr constant, then the value of e 

 would be the same throughout the insulating material of the 

 cable and every portion of the insulating material would carry 

 its full share of the total stress (voltage) between wire core and 

 sheath. Such a gradation of k is however, impracticable, but 

 it is practicable to apply the insulating material in two or three 

 layers, the inside layer having large inductivity, the middle layer 

 having medium inductivity, and the outer layer having small 

 inductivity. Such a cable insulation is called a graded cable 

 insulation. 



96. Magnetic and electric parallels, 

 is analogous to electric inductivity. 



Magnetic permeability 



The lines of force of a mag- 

 netic field converge upon and 

 pass through an iron rod as 

 shown in Fig. 102. 



The lines of force of an elec- 

 tric field converge upon and 

 pass through a glass rod as 

 shown in Fig. 103. 



Fig. 102, 



Fig. 103, 



