16 BELL SYSTEM TECLINICAL JOURNAL 



is exactly like every other part lia\ing the same length. If the line 

 is loaded with coils then each luaiiing section is exactly like e\cr\- 

 other loading section. 



When a telephone transmitter or other signaling device A acts 

 upon such a line it causes a wave to travel over the line away from 



1-ig. 1 



the source. If the line includes resistance or other losses liiis wave 

 gradually becomes smaller until it is too weak to be detected but no 

 portion of the wave returns to the source after once leaving it. 



If some portion of the line difTers in its electrical makeup from 

 other portions of the line it constitutes an irrcgularit\- and interferes 

 with the passage of the wa\e. 



Fig. 2 shows a line exactly like that of Fig. 1 except that an irreg- 

 ularity B has been introduced. This irregularity has been shown 



I-ig. 2 



as a series resistance though any other departure from the regular 

 electrical structure of the line would produce similar efTects. 



When a wa\e encounters such an irregularity, it splits into two 

 parts one of which continues in the original direction of propaga- 

 tion along the line while the other is propagated in the opposite 

 direction toward the source. 



In order to understand this phenomenon, which is called reflection, 

 imagine that a wave is traversing the line from left to right. As it 

 passes the point B a current flows through the series impedance 

 which constitutes the irregularil\- and this causes a drop of potential 

 through the imjicdance. ()b\ i((Usl\-, this changes the state of alifairs 

 as there is now a sudilen alleralion in the \-oltage across the line as 

 the wa\e ])asscs llie irregularity wlurias there is no such alteration 

 without the irregularity. 



Suppose that for the impedance element we substitute the output 

 terminals of a generator which has a negligible impedance and arrange 

 the generator so that it is excited i)y the wave tra\cling over the line 

 but that the excitation is not afTected by the voltage set up by the 

 generator itself. Such an arrangement is shown in Mg. 3. The 



