270 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



a telephone line suffers continual distortion because of the rapid 

 decay of magnetic field due to line resistance. The distortion of 

 an electric wave as it travels along a pair of telephone lines is 

 similar in many respects to the distortion of a canal wave as de- 

 scribed in Art. 144 and as represented in Fig. 194. Imagine a 

 rectangular electromagnetic wave-pulse to be started at the middle 

 of a long telephone line (two wires of course). Let the small 

 rectangle in the upper part of Fig. 194 represent the initial form 

 of the wave. After the elapse of time the wave changes to the 

 shape shown by BB, Fig. 194. The energy in the head of the 

 wave decreases partly because of the RI Z losses in the line wires 

 and partly because of the shooting of energy back into the tail 

 of the wave. 



The transmission of articulate speech over a telephone line de- 

 pends upon the transmission of characteristic shapes of electric 



waves. Thus, the shapes of the 

 electric waves necessary to re- 

 produce certain vowel sounds are 

 shown in Fig. 202, and the wave 

 shapes which are necessary to 

 produce consonant sounds are 

 very much more complicated than 

 these. The wave distortion on 

 the line tends to make each ele- 

 mentary portion of a wave spread 

 out as shown in Fig. 194, and if 

 each elementary portion of a com- 

 plicated wave spreads out in this 

 way the fine details of wave shape 

 are very soon obliterated as the wave travels along. 



It is not desirable to eliminate wave distortion by providing 

 poor insulation between telephone wires because this results in a 

 great reduction in the amount of energy transmitted. The method 

 which is used in practice is to connect small inductance coils in 

 circuit with the line wires at intervals over the whole length of 



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Fig. 202. 



