Practical Application of Carrier Telephone and Telegraph in the Bell System

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 announced that the engineers of Bell INhad1918 it was carrier current telephone apparatus the such aSystem perfected to point 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 " c a r r y " the telegraph or telephone messages.