TELEPHONE TRANSMISSION OVER LONG CABLE CIRCUITS 79 



without serious distortion.) It is also necessary to limit this power 

 to avoid serious crosstalk into other circuits. 



In addition to these limitations on the maximum power, it is neces- 

 sary to insure that the power at any point in a circuit does not become 

 too small. Otherwise, the normal voice power will not be sufficiently 

 large as compared to the power of crosstalk from other circuits. It 

 is, furthermore, evident that the ratio of power from extraneous 

 sources, such as paralleling telegraph circuits and power supply cir- 

 cuits, to the voice power should be as small as practicable in order to 

 keep the circuits free from noise. 



Figure 7 will give an idea of how the telephone power attenuates 

 and is amplified in a long circuit. The circuit shown is similar to 

 those which it is proposed to employ between New York and Chicago, 

 i. e., it is a four-wire X.L.L. 19 gauge circuit largely in aerial cable, 

 equipped with automatic means for compensating for the changes 

 in attenuation caused by the effects of varying temperatures on the 

 resistance of the conductors. (These automatic devices are described 

 in a later section of this paper.) For simplicity, the power levels for 

 transmission in one direction only are shown. The solid lines show 

 the power levels when the temperature is a maximum so that the 

 attenuations are greatest, while the dotted lines show the levels when 

 the temperature is a minimum and the losses are, therefore, also a 

 minimum. The shaded areas between the lines represent the changes 

 which take place during the course of a year. 



When the requirement is introduced that transmission must take 

 place in both directions it is found that at the points in the circuits 

 going in one direction where the power is a maximum, the power going 

 in the opposite direction in other circuits is a minimum. This rep- 

 resents a very bad condition for crosstalk from one four-wire circuit 

 into another. In order to overcome this the conductors carrying 

 strong voice power are kept electrically separated or shielded from 

 those carrying weak power as indicated schematically in Figure 8. 

 The conductors which carry strong voice power are shown heavy, 

 while those carrying weak power are shown light. In the cable proper 

 the separation is effected by grouping the conductors in two bunches, 

 one for transmission in one direction, the other for transmission in 

 the opposite direction, taking care that these two bunches of con- 

 ductors are separated electrically as far as possible. In the loading 

 coil pots the coils employed on the circuits for transmission in the 

 two directions are similarly kept separated. In the offices the separa- 

 tion is effected by arranging the repeaters and other apparatus as 

 shown in the figure. It will be observed that no special separation 



