An analysis of telephone currents shows that, in order to transmit 

 satisfactory speech, it is necessary for all of the telephone ap|Kiratus 

 and circuits involved to transmit with nearly uniform efficiency simple 

 alternating currents over a considerable range of frequencies. For 

 new designs of telephone circuits, the minimum range so transmitted 

 is between about 250 and 2,750 cycles. The voice contains com- 

 ponents of lower frequencies and also of high frequencies but it is not 

 necessary to transmit them because their contribution to the clearness 

 of the speech is relatively unimportant. 



Fig. 7 — Graph of a simple alternating current or "sine wave." 



A similar analysis of the currents used for telegraphic transmission 

 shows that they may be considered also as composed of simple alternat- 

 ing currents covering a band of frequencies — with equipment generally 

 used in the Bell System, this band extends from zero up to roughly 

 about 100 cycles. Components of the frequencies above about 100 

 cycles can be excluded from the telegraph circuit without reacting 

 upon its efficiency of transmission with the equipment and the speeds 

 of signaling commonly employed in private line telegraph circuits. 

 This difference in the range of frequencies required for the trans- 

 mission of telephone currents and for the transmission of telegraph 

 currents makes possible the application of the principle of separation 

 of electrical currents into different channels depending on the differ- 

 ence in their frequency of alternation mentioned above. Apparatus 

 placed at the terminals of the circuits, which is called "composite sets," 

 is so designed that telegraph currents and the telephone currents can 

 be transmitted into the same wires and at the receiving end are sepa- 

 rated into the telephone and telegraph channels, respectively, without 

 interference. The form of this apparatus is indicated diagrammati- 

 cally in Fig. 8. 



The principles of simplexing and compositing have been applied 

 extensively to the long distance circuits of the Bell System, there being 

 now in service approximately 760,000 miles of telegraph circuit oper- 



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