ADVANCES IN CARRIER TELEGRAPH TRANSMISSION 163 



ability of large numbers of composited and simplexed circuits, most of 

 which were used in private line service/ and the low traffic density on 

 many long multi-section circuits, making it desirable to provide 

 intermediate operating points. By about 1920, the weight of evidence 

 definitely favored the multiplex method of exploitation over the use 

 of the high-speed printer.'' It was at about this point in telegraph 

 history that the carrier telegraph method of subdividing the line 

 capacity made its appearance and, through its superior flexibility and 

 lesser intricacy of operation, began gradually to supersede the dis- 

 tributor methods of multiplexing circuits for many types of services. 



While voice-frequency telegraphy was foreshadowed by Elisha 

 Gray's harmonic telegraph,^ which was exhibited at the Third French 

 International Exposition in 1878 and at the Electrical Exhibition in 

 Paris in 1881,^ its practical embodiment had to await the invention of 

 the electrical filter by Campbell, that of the audion by DeForest, and 

 the production of effective means for generating alternating currents 

 of acoustic frequencies. 



The success of this system rests mainly upon its adaptability to 

 economical operation over telephone circuits by making effective use 

 of the whole frequency band usually allocated to the voice, and in 

 requiring similar transmission characteristics. Henceforth, every 

 advance in telephony directed to an improvement of the transmitting 

 medium contributes as well towards the improvement of telegraphy; 

 the economies of wide-band carrier telephony, the improved equaliza- 

 tion and regulation of circuits, the reduction of interference and the 

 elimination of crosstalk, all tend to make the telegraph a more de- 

 pendable and efficient tool for modern industry and modern living. 

 Thus telegraphy, one of the oldest of the electrical arts, having fathered 

 the telephone, now finds, within the great technical structure which 

 the latter has created, a fertile medium for the development of its 

 usefulness, not as a competitive but as a complementary service. 

 Thanks to voice-frequency telegraphy, wherever the telephone reaches, 

 a high-speed, reliable, record-form of telegraph may follow. This has 

 brought about a great simplification of the problem of interconnection 

 in such large communication networks as the international postal area 

 in Europe and the Bell System in our own country. 



Furthermore, carrier telegraphy has doubtless been a means of 

 advancing the fortunes of the start-stop teletypewriter,'" by subdivid- 

 ing the frequency band to such an extent that one channel may be 

 economically assigned to a single operator working at normal speed. 

 It has also been a factor in simplifying the switching problems pre- 

 sented by the extensive introduction of teletypewriter exchange 

 (TWX) service. 11 



