ELECTRIC CIRCUITS APPLIED TO COMMUNICATION 15 



Long Line Phenomena 



The notable success of the vacuum tube ampHfier has made a great 

 change in the character of the problems encountered in the design of 

 very long telephone circuits, and our discussion would not be complete 

 without a brief consideration of the nature of these problems. A 

 detailed discussion of these problems is given in numerous papers in 

 recent technical literature. 



With the amplifier it is possible in a very large measure to overcome 

 at a relatively small cost the effects of power losses in the telephone 

 circuit. Whereas before the general use of the amplifier it was 

 necessary to make every possible effort to further improve the efficiency 

 of the long circuits in order to increase the distance over which satis- 

 factory telephone transmission could be given, with the amplifier of 

 today there is a limit to the amount of money which can properly be 

 spent merely for the improvement of the a olume efiiciency of the line 

 as the line losses can always be made up if desirable by the use of 

 amplifiers. As a result, in general, very long telephone circuits have 

 become electrically so long that factors other than power efficiency 

 determine the limits of their effectiveness. While a quantitatiAe 

 theoretical discussion of these problems is necessarily in large measure 

 beyond the scope of undergraduate work at the present time, this may 

 not long be the case, and in any event a general appreciation of these 

 phenomena is of a good deal of interest. 



Although these effects are common to all long circuits in principle, 

 they are most prominent in very long telephone cable circuits as these 

 are electrically the longest circuits in use. For example, the propaga- 

 tion constant of a toll circuit in cable between New York and Chicago 

 is at 1,000 cycles approximately 50 + j300. This is approximately 

 the same as the propagation constant of a high voltage power line 

 transmitting power at 60 cycles of 25,000 to 50,000 miles in length. 

 If there were no intermediate amplification in the circuit the ratio of in- 

 put to output power would be ten to the 45th power so that with our 

 usual telephone input of about one milli-watt the circuit would deliver 

 only one electron in each two months, and even if all the power avail- 

 able in New York City or Chicago could be used at the input without 

 burning up the circuit, the received current would be utterly inappre- 

 ciable. However, there are far more practical reasons than this for 

 frequent intermediate amplification. The lower limit to which the 

 power level can be permitted to fall in the circuit is limited by the 

 disturbances picked up from other telephone circuits in the same cable 

 or from other electric circuits outside the cable, and the maximum 

 power level is, of course, limited by considerations of economy in the 



