154 BELL SYSTEM TECIISUAL JOLKX.-IL 



and then substituting for the line a non-inductive resistance until 

 the same value of current is drawn from the oscillator. In employing 

 this method for determining the surge impedance it was assumed that 

 the oscillator output was constant, and that the phase angle of the 

 surge impedance was small. 



A study of the curves on Fig. 1 shows that the apparent impedance 

 of the line will change with the impedance in which the line is termi- 

 nated in different ways, depending upon the frequency used. (1) If 



MEASURED IMPEDANCE - OHMS 



Kig. 2 — Graphical Solution of Substitution Methotl for Determining the Surge 

 Impedance of a Power Line 



a frequency mid-way between the quarter wave lengths* is used, 

 the open circuit and short-circuit impedances are the same. (2) If 

 a fre(|uency corresponding to an e\en quarter wave length is used, 

 an increase in the terminating impedance will produce an increase 

 in the ajiparent im()edance f)f the line. (3) If a frequency corre- 

 sponding to an cxid quarter wave length is usetl, an increase in the 

 terminating impedance will produce a decrease in the apparent impcd- 



' Whenever the length of the line Ixjcomes equal to, or some multiple of, one 

 quarter o( the length of the eli-ctric wa\e of the corresponding frequency, it is referred 

 to as a quarter wave length fre<|uency, or, for short, a quarter wave length. 



