ELECTRIC DISTRIBUTION AND WIRING. 



301 



Maximum electric stress in the insulation of a cable. Let R l be the radius of 

 the central wire of a cable and R^ the inside radius of the sheath. The electric 



Fig. 171. 



field intensity (volts per centimeter) at a point/, Fig. 171, distant x from the axis of 

 the cable, is proportional to \\x. Therefore we may write 



wherey is the electric field intensity at a point distant x centimeters from the axis, and 

 k is a constant to be determined. The electromotive force, , between the central 

 wire and the sheath is : 



/.#2 



E= f.dx (ii) 



'*, 



whence, substituting the value of f from (i), we have : 



k= 



E 



Therefore : 



Now the electric field intensity at the surface of the central wire is k\R-^ from equation 

 (i) or, using the value of k from (iv), we have 



/max. = 



R 



. 



(48) 



in which yjj iax . is the maximum electrical stress (volts per centimeter) in the insula- 

 tion of a cable, .A*j is the radius of the central wire in centimeters, R,,, is the inside 



