up the voice currents are not all transmitted with the same velocity 

 over the circuit. Often the frequencies in the middle of the range 

 1,000 to 1,500 cycles arrive first and the highest and lowest frequencies 

 arrive somewhat later. This difference is inappreciable on short 

 circuits but for the longest circuits, if not corrected for by suitable 

 design, may become great enough to be appreciable. Under those 

 conditions a distortion of the speech takes place which interferes with 

 the ease of understanding and, in extreme cases, may seriously impair 

 transmission. 



This type of effect can be compensated for by the installation at 

 intervals along the circuit of networks designed to introduce addi- 

 tional delay in the transmission of the frequencies in the middle of the 

 range so that all frequencies will arrive at the distant end more nearly 

 at the same time. Up to the present time, the improved design of 

 circuits used for the very long distances has sufficiently kept down the 

 amount of this distortion so that special compensating arrangements 

 are not necessary to message circuits but they are commonly used in 

 circuits for some of the special services, where transmission require- 

 ments are more severe. 



Still a third effect of the finite velocity of transmission over telephone 

 circuits is to be found in the time of transmission itself. In the 

 ordinary case, the elapsed time between the speaking of a word at one 

 end of the circuit and its reproduction at the distant end is inappreci- 

 able but for very long circuits this requires consideration. Telephone 

 conversations, like face-to-face conversations, involve the repeated 

 interchange of information. Even if one person is doing the talking, 

 he receives frequent acknowledgments from the other that he is 

 followed and understood and, in the case of telephone conversations, 

 those acknowledgments must be vocal in character. If too great a 

 time is required for the transmission of the speech and the return 

 transmission of the acknowledgment or replies, the vocal interchange 

 of ideas is interfered with. 



These considerations have led to the preliminary conclusion that the 

 total time of transmission over any telephone circuit should not exceed 

 about 3^ of a second. It would mean that the velocity of transmission 

 20,000 miles a second now used for long toll cable circuits would not be 

 adequate at some future time for connections between widely separated 

 parts of the earth's surface. Fortunately, the trend of development 

 of very long circuits is for various reasons in the direction of higher 

 velocity circuits, as will be made apparent in the next section, so that 

 it is anticipated that this limitation, except in perhaps a few special 

 cases, will not be difficult to overcome. 



29 



