CHAPTER XVI. 



TRANSMISSION LINES.* 



156. Direct-current and alternating-current line calculations com- 

 pared. A transmission line is generally designed to deliver a 

 certain amount of current to a receiving circuit at a specified 

 voltage, the difference between generator voltage and receiving 

 circuit being given. In the case of direct-current transmission, 

 this problem is solved by considering the resistance drop RI in 

 the line, as fully explained in Chapter IX of the first volume of 

 this text. In the case of alternating current, an exact solution of 

 the problem would require the effect of line resistance, of line 

 inductance, and of line capacity to be taken into account. The 

 effect of line capacity, however, is negligible on moderately short 

 lines and it is ignored in this chapter, f 



In the case of alternating current, the product of line resistance 

 by the effective value of the line current is called the resistance 

 drop in the line, the product of the line reactance by the effective 

 value of the line current is called the reactance drop in the line, 

 the product of the line impedance by the effective value of the 

 line current is called the impedance drop in the line, and the 

 numerical difference between generator voltage and receiver 

 voltage is called simply the line drop. The general relation 

 between resistance drop, rl y reactance drop, xl, and the impe- 

 dance drop, zl is shown in Fig. 79, Chapter IV and it is evident 

 that the line drop (difference in numerical values of Q and E^) 

 depends not only upon the value of the impedance drop 

 (/1/r 2 + .# 2 ), but also upon the phase relations as shown in Fig. 79. 



157. Line resistance. The resistance of a wire for alternating 

 current is in nearly all practical cases equal to the resistance of 



* See a paper by F. A. C. Perrine and F. G. Baum, Transactions Am. Inst. E. E. 

 Vol. XVII, pages 345-377 ; also a paper by F. G. Baum, Trans. Int. Elec. Cong. Vol. 

 II, pages 243-278, St. Louis, 1904. 



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