14 BELL SYSTEM TECHNICAL JOURNAL 



transformers at the ends of the Hne, the line itself having a relatively 

 small efifect on the impedances thus obtained. For the longer lines, 

 however, the line characteristics play a very important part in this 

 process, and this is best illustrated by consideration of the telephone 

 case. 



In the ordinary telephone line the leakage losses are also small 

 compared with the resistance losses, and the voltage can be raised by a 

 factor of 2 or 3 before these losses become equal. The solution of the 

 problem here as in the case of the power line is therefore to raise the 

 transmission voltage and decrease the current, that is, to increase the 

 impedance of the line. In a telephone case, however, this cannot be 

 done by changing the impedance of the terminal apparatus since, as 

 has been pointed out, practically all of the power is absorbed in line 

 losses, and therefore the impedance at the transmitting end of the line 

 is not appreciably affected by the impedance of the receiving apparatus. 

 In order to raise the ratio of voltage to current on the telephone line, 

 it is therefore necessary to operate on the line itself. 



The impedance of an electrically long transmission circuit is very 

 approximately equal to -ylL/C. In telephone lines the most practical 

 way to increase this impedance is to increase the inductance. This 

 may be done, as you know, either by uniformly distributed inductance 

 or by lumped inductance, providing certain essential conditions are 

 met, and the result is what is called a "loaded" telephone circuit. It 

 is particularly to be noted that this is not a resonance phenomenon. 

 On the contrary, loading in this way tends to decrease the variations 

 with frequency of the efficiency of transmission of the circuit, and when 

 so proportioned as to give maximum power efficiency, results in distor- 

 tionless transmission. 



In the very long power lines on the other hand, it is desired to trans- 

 mit efficiently only one frequency, the fundamental. The use of 

 methods depending upon resonance is therefore permissible, and in 

 fact the method which will undoubtedly have increasing use in the 

 future as with increasing length of power transmission lines the effects 

 of line capacity become more important, will be the partial neutraliza- 

 tion of the effects of this capacity by shunt inductances distributed 

 along the line, that is, induction machines, or synchronous machines 

 underexcited. This reduces the equivalent capacity of the lines by 

 supplying at least a part of the charging current at the intermediate 

 points. Thus is a similar end obtained in the two cases by different 

 means. 



