84 BELL SYSTEM TECHNICAL JOURNAL 



(23 kilograms per kilometer) loaded with inductance coils of 0.2 henry 

 spaced 1.4 miles (2.25 kilometers) apart and contained 6 one-way 

 repeaters. It will be noted that the first sign of the arrival of the 

 received current occurs about 0.1 second after the wave is put on at 

 the sending end. This time checks with the formula for velocity 

 given above. The time required after arrival of the first impulse 

 (point "a") until the wave builds up to a practically steady-state 

 condition at point "b" is about 0.055 second. The steady condition 

 is interrupted at point "c" by the arrival of the break transient, the 

 time interval between points "b" and "c", representing the period 

 when the wave is in the steady-state, being only 0.01 second. The 

 wave required about 0.055 second to die out — interval between points 

 "c" and "d". 



It is interesting to note the behavior of the current during the 

 building-up and dying-out intervals. During the building-up process 

 the frequency of the received current increases from a very low value 

 at point "a" until at point "b" it becomes the same as that of the 

 source. The magnitude of the received current also increases until 

 at point "b" it reaches a value corresponding to the steady-state 

 transmission equivalent of the line. The interval "a-b" is determined 

 solely by the structure of the line and has nothing to do with the time 

 during which the current is supplied at the sending end. 



The dying-out process can be considered to be caused by the applica- 

 tion at the time of break of a second current equal in value to the 

 current originally applied but opposite in phase, so that the sum of 

 the two currents will be zero. Hence, it is to be expected that the 

 received current will disappear by adding to the steady-state a tran- 

 sient similar to the building-up transient in the interval "a-b". That 

 this is true is indicated by the behavior during the interval "c-d". 

 At first the low frequency current of the break transient produces a 

 displacement of the axis of the steady current. As the frequency 

 approaches a steady value a beating effect becomes noticeable which 

 grows smaller until complete opposition of phase obtains and the 

 received current disappears. 



Figure 10 clearly indicates that a pulse of voice current having a 

 frequency in the neighborhood of 1800 cycles, even though received 

 in proper volume if steadily applied, would be badly distorted. 



When carrying on a conversation over such a circuit as this, distor- 

 tion of the voice waves makes understanding difficult while peculiar 

 metallic ringing sounds are very noticeable. 



Next consider a circuit of the same character with half the length. 

 The effect of a circuit of this length on an 1800-cycle wave is shown in 



