300 ELEMENTS OF ELECTRICAL ENGINEERING. 



The voltage determines the average intensity of the electric 

 field between the core and the casing, and the figure shows 

 that the electric field is much more intense near the surface 

 of the small wire in B than it is near the surface of the 

 large wire in A. Therefore the strength of a given insulating 

 medium to withstand the electric stress between the core and 

 sheath of a cable depends not only upon the radial thickness of 

 the insulation but also upon the radius of the central wire. 



Fig. 170. 



Each of the diagrams, A and B, Fig. 1 70, represents in section 

 the two wires of a transmission line, and the fine curved lines rep- 

 resent the lines of force of the electric field in the insulating 

 medium for the same voltage between the wires in each case. 

 The voltage determines the average intensity of the electric field 

 along the straight lines, //, and the figure shows that the electric 

 field is much more intense near the small wires in B than it is 

 near the large wires in A. Therefore the strength of a given in- 

 sulating medium to withstand the electric stress between two 

 line wires depends not only upon the distance from surface to 

 surface of the wires, but also upon the size of the wires. 



This dependence of the strength of the line insulation upon 

 the size of the wires may be made more intelligible in terms of a 

 mechanical analogue. Thus a block of glass is much more 

 severely strained when it is squeezed between two fine-pointed 

 tools than when it is squeezed with the same total force between 

 two blunt-pointed tools, and it is in the immediate neighborhood 

 of the fine points that the excessive strain occurs. 



