TELEPHONE TRANSMISSION OVER LONG CABLE CIRCUITS 85 



the oscillogram of Figure 11. It will be observed that the propaga- 

 tion time has been cut in half while the lengths of time for the received 

 wave to build up and die out have also each been cut in two. This 

 checks with theoretical work, indicating that the severity of this type 

 of transient effect is directly proportional to the length of the circuit. 

 This fact that the transient effect is proportional to the length of 

 the circuit furnishes the reason why a short circuit may give tolerably 

 good results, while a long circuit gives poor results. 



Figure 12 is of interest as indicating what takes place when we apply 

 a current at the sending end of the circuit whose frequency is so high 

 that no appreciable amount of the steady current will pass through 

 the circuit. In this case only transient oscillations appear at the 

 receiving end of the circuit. This particular circuit was of the same 

 type as the above, although it was only 350 miles long (570 kilometers). 



A large number of oscillograms of this sort have been taken in con- 

 nection with the study of these transient effects. From these and 

 theoretical considerations 3 it has been proved that the effects in a 

 given circuit are much worse at high frequencies than at low fre- 

 quencies, the severity of the effects, within certain limits, being a 

 function of the ratio of the frequency being transmitted to the fre- 

 quency of cutoff of the loaded circuit. The gauge of the circuit has 

 practically no effect. 



Since in order to give good quality it is necessary to transmit fairly 

 well all frequencies up to at least 2000 cycles, it is obvious that on 

 long circuits in order to keep the transient effects small, the frequency 

 of cutoff must be kept high. In order to do this, it is necessary either 

 to make the loading coils of very low inductance or to space them very 

 close together. This is another one of the reasons why extra light 

 loading was adopted for the long cable circuits. (It will be remem- 

 bered that the inductance of the side circuit loading coils is only 0.044 

 henry and the spacing 6000 feet). 



The effect of lighter loading on the transient behavior of telephone 

 currents, is shown in Figure 13, which shows a 2000-cycle wave 

 transmitted over an X.L.L. circuit about 1050 miles (1700 kilometers) 

 long. This circuit contained 23 one-way repeaters. It will be 

 observed that both the building-up and dying-out transient periods 

 are very much reduced, which means that all pulses of telephone 

 currents up to at least 2000 cycles will pass through such a circuit 

 with very little distortion. 



3 John R. Carson — "Theory of the Transient Oscillations of Electrical Networks 

 and Transmission Systems". Transactions of A. I. E. E. Vol. XXXVIII, page 407. 



