56 BELL SYSTEM TECHNICAL JOURNAL 



listener at the other end will hear the speech in proper time relation, 

 independent of how much absolute delay there is in going from one 

 end of the circuit to the other. However, when the speaker asks a 

 question and waits for the answer, the slowing-down effect on his con- 

 versation will evidently be the time of transmission of his question to 

 the distant end and the transmission back from the distant end of the 

 answer. Considering the speed shown in the table, however, consider- 

 ation may be taken of the fact that the non-loaded constructions, both 

 open wire and cable, and the radio, are of such high speed that con- 

 versations could be carried on over them for the longest distance be- 

 tween places in the world without appreciable difficulty. This, how- 

 ever, is not the case for the loaded construction. Assume, for example, 

 that a length of 4,000 miles would cover the wire line distance between 

 any two points in this country. For the slowest construction noted, 

 an interval of 0.8 second is required for transmission to the distant end 

 and return. While it is possible to carry on conversation over a circuit 

 with this delay, it is larger than is considered desirable. The faster 

 of the loaded constructions shown would give a delay over circuits of 

 this length which represents somewhere about the limit of what, at the 

 present time, is considered satisfactory. Incidentally, the slowest 

 of the constructions shown, for this and other reasons, is not proposed 

 for use except for comparatively short distances. 



Communication engineers must look forward to the time when the 

 longest cable distances in North America are connected, in some cases by 

 submarine cable, to long lengths of cable in Europe. With this as an 

 ultimate objective, this matter of the direct effect of delay on conver- 

 sation has become of considerable interest. 



An appreciation of the transmission time on long telephone circuits 

 may perhaps be gained by considering the distance required to produce 

 an equivalent delay of sound waves traveling in air. For example, it 

 takes about as long for a radio wave to travel half way around the earth 

 at the equator as it does for a sound wave to travel from one speaker 

 to another when the distance separating them is about 75 ft. Inci- 

 dentally, the time required for a radio wave to travel from the earth 

 to the planet Mars would be from about 3 to 20 minutes, assuming that 

 it got there at all. Evidently if we have neighbors on Mars we can 

 never hope to carry on conversation with them. 



Telephone engineers have devoted considerable attention to the 

 effects of delay on the telephone users in an effort to determine how 

 far the electrical waves should be permitted to travel over different 

 types of circuits. Since the present constructions do not offer any 

 particular difficulties, for the distances now in use, a look into the fu- 



