CHAPTER II. 



ELEMENTS OF A TRANSMISSION LINE. 



THE essential elements of a transmission line have been 

 described many times, but a short discussion of them, 

 with an explanation of some of the terms used in connec- 

 tion with the subject, may be useful before proceeding 

 with the actual calculations. 



A transmission line consists of two or more conductors 

 insulated from each other so that they can carry energy by 

 electric currents to some more or less distant point. 



The conductors may be solid copper wires, copper cables, 

 or aluminum cables. The diameters and resistances of 

 various standard conductors are given in Tables VII and 

 VIII, pages 118 to 120. It will be noted that the exact 

 value of the resistance of a conductor differs slightly when 

 a direct current, and an alternating current of 25 or 60 

 cycles, is flowing. This is due to the "skin effect/' by 

 which an alternating current tends to flow near the surface 

 of a conductor, as explained in Chapter X. The drop in 

 voltage due to resistance is proportional to .the current 

 and is in phase with it when the current is alternating. 



Only overhead lines, carrying alternating currents, will 

 be considered in this book. Such lines are supported by 

 poles or steel towers at a considerable height above the 

 ground. The conductors are separated from each other by 

 a distance which may be several inches or several feet. 

 This distance is called the "spacing" of the conductors 



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