Practical Application of Carrier Telephone and 

 Telegraph in the Bell System 



By ARTHUR F. ROSE 



IN 1918 it was announced that the engineers of the Bell System 

 had perfected carrier current telephone apparatus to such a point 

 that four talking circuits had been added to one pair of wires already 

 in use for telephone and telegraph communication and were being 

 used commercially between Pittsburgh and Baltimore for providing 

 needed telephone facilities. Since that time the growth of carrier 

 application in the Bell System has been quite rapid. The purpose 

 of this paper is to summarize the applications of carrier up to the 

 present time and give a few typical examples where it has been found 

 economical to provide circuits by means of carrier rather than by other 

 types of facilities. 



Principles of Operation 



The theory of carrier current systems, together with a historical 

 sketch, was presented by Messrs. Colpitts and Blackwell before the 

 American Institute of Electrical Engineers in February, 1921, and 

 was published in Volume XL of the Transactions of the Institute. 

 For those who do not wish to go into the detailed theory given in that 

 paper, it may suffice to say that in a carrier current system a number 

 of telephone or telegraph messages are simultaneously superposed on 

 a single pair of wires by means of high frequency currents of different 

 frequencies on which the individual messages are impressed. It 

 is from this principle that the carrier current systems get their name, 

 as the individual high frequency currents may be said to "carry" 

 the telegraph or telephone messages. By using different frequencies 

 for the carrier currents, the individual messages retain distinctive 

 features which enable them to be separated one from another at the 

 receiving end of the circuit. 



On account of the much higher frequencies that are used in carrier 

 operation, the carrier currents are attenuated more rapidly than the 

 ordinary low frequency voice currents. This requires that repeaters 

 be located at frequent intervals in a carrier system. In these re- 

 peaters all the carrier channels are amplified together although the 

 ordinary voice frequency channel is separated out and amplified in 

 its own repeater. 



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