16 BELL SYSTEM TECHNICAL JOURNAL 



design of the amplifiers. These limits result in the use of amplification 

 in these circuits at approximately 50 mile intervals, there being 17 

 intermediate points of amplification between New York and Chicago 

 on the shortest route. 



With such a circuit the variation of efficiency with temperature is 

 very rapid and the emergence of the sun from under the clouds could 

 make as much as 1,000 fold-difference in the amount of power delivered. 

 It is therefore necessary for practical operation to control the power 

 gain introduced by the amplifiers by means of pilot wires in the cable 

 subject to the same temperature variations as the talking circuit, and 

 in this way to compensate automatically for the effect on the propaga- 

 tion constant of varying temperature. 



In order to obtain for these cable circuits as far as possible the bene- 

 fits of high voltage transmission, the circuits are loaded. This loading 

 also in large measure equalizes the efficiency of transmission over the 

 frequency range required for the transmission of speech. 



The velocity of wave propagation over conductors loaded with in- 

 ductance is, of course, relatively slow compared with the velocity of 

 light. In the case of loaded telephone circuits in cable the velocity for 

 the two types of circuit in general use is respectively 10,000 miles a 

 second and 20,000 miles a second. The low velocity circuits are loaded 

 with more inductance and are of higher efficiency and therefore pre- 

 ferable from the standpoint of volume. It is necessary, however, for 

 the long circuits to use facilities of higher velocity and lower efficiency 

 because of several very interesting phenomena. 



For one thing, with circuits of high efficiency conforming to present 

 day standards, the currents reflected from the distant end because of 

 irregularity of impedance between the line and the terminal apparatus 

 are by no means inappreciable. When the time for the propagation of 

 these currents over the line and back to the transmitting end is very 

 short, the reflected currents can be large without interference with 

 service as they are indistinguishable from the sound directly heard by 

 the speaker. If a circuit from New York to Chicago were used on the 

 lower speed of the two types of circuit mentioned above, however, the 

 reflected currents would arrive about one fifth of a second late. An 

 interval of this magnitude would result in serious confusion to the 

 speaker due to hearing his words twice, by direct transmission and 

 after reflection from the distant end of the circuit. With the high 

 speed facilities the time interval is reduced to one tenth of a second, 

 and the interfering effect is very much smaller. Even with the high 

 speed facilities, however, the effect is sufficient so that on circuits of 

 over a few hundred miles in length special devices known as echo 



