TRANSMISSION LINES. 



347 



the same wire for direct current. When the wire is very large, 

 however, or when, the frequency is very high, the alternating 

 current near the axis of a wire lags in phase behind the current 

 near the surface of the wire, and the resistance of the wire is 

 therefore larger for alternating current than for direct current. 

 This effect is called the skin effect* 



158. Line reactance. The reactance of a transmission line 

 (outgoing and returning wires side by side) depends upon the 

 size of the wires and upon their distance apart center to center, 

 and it is proportional to the length of the line and to the fre- 

 quency. f The following table gives the resistance and reactance 

 per half-mile of transmission line : 



RESISTANCE AND REACTANCE OF ONE MILE OF WIRE 

 OF TRANSMISSION LINE) (EMMET). 



MILE 



159. Calculations of a single-phase transmission line to give a 

 specified line drop.J A single-phase transmission line is to 

 deliver a prescribed amount of power P at a prescribed electro- 



* See Merritt, Physical Review, Vol. 5, page 47. 



| The formula for the reactance of a line is developed in Appendix A, and so also is 

 the formula for line capacity. 



J See a paper by R. D. Mershon, American Electrician, June, 1897. 



