PART II. 

 THEORY. 



CHAPTER VII. 



CONDUCTORS. 



THREE main classes of conductors are used for overhead 

 lines for the transmission of electric power; namely, copper 

 wires, copper cables and aluminum cables. The cables used 

 are generally strands of seven wires; that is, they consist 

 of a central straight wire with six 

 wires wound spirally around it, as 

 indicated by the cross section in 

 Fig. 9. 



From this figure it is seen that 

 the maximum diameter of a y-wire 

 strand is equal to 3 times the di- 

 ameter of one of the wires. 



The outside wires do not follow ~2f" 



a straight path parallel to the Fig ' 9 ' 7 ' wire strand ' 

 central wire and the axis of the cable, but lie in a spiral 

 around it, as mentioned above. As there is always a 

 slight insulating film of oxide on any wire, the current 

 flowing in the cable tends to stay in the individual wires, 

 and so follows the longer path. Thus, the resistance of a 

 cable is greater than that of a solid wire of the same area 

 of cross section. The amount of the difference depends on 

 the number of wires in the cable and the pitch of the 



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