PRACTICAL APPLICATION OP CARRIER SYSTEMS 49 



within the next few years but in the meantime, the use of carrier 

 apparatus enables the traffic growth to be taken care of without 

 stringing wire which, under the circumstances, would be very expensive. 



There are, at present, operating between Harrisburg and Chicago, 

 one 4-channel telephone system and between Pittsburgh and Chicago, 

 two 3-channel systems, providing a total of 10 carrier telephone 

 channels. These will be supplemented by at least two additional 

 systems before the cable is completed. As soon as the cable is in- 

 stalled the carrier systems will probably be removed from service 

 here and reinstalled in other locations. 



In most cases the problem of providing additional circuits is not as 

 difficult as in the section between Pittsburgh and Chicago. For 

 this reason the relative economies of providing circuits by carrier 

 and by the other methods must be more carefully considered. Even 

 where no congestion exists, however, it will be found that where the 

 circuits are long enough the carrier circuits will be cheaper than any 

 other method of providing the facilities. The circuits to be provided 

 must usually be several hundred miles in length before this is the 

 case; also, since the cost of a carrier channel goes down as the number 

 of channels installed at one time is increased it will usually be found 

 that an installation of 3 channels will prove in for considerably shorter 

 distances than would be necessary if a lesser number of channels are 

 installed. In the practical case a complete system consisting of 

 either three or four channels is usually installed at one time. 



A typical case of a carrier telephone installation where the existing 

 open wire lead is not already full but where the circuits required are 

 long, is the Oakland-Portland system which in conjunction with a 

 short cable between Oakland and San Francisco provides San Fran- 

 cisco-Portland circuits. The detailed layout is shown on Fig. 6. 

 Here the cost study showed a considerable saving in annual charges 

 in favor of the carrier although there was room for stringing open wire 

 on the existing pole line. This system was put in service by the 

 Pacific Telephone and Telegraph Company in 1921 and has since 

 given very satisfactory service. 



Another type of carrier installation is one installed to defer a pro- 

 posed cable project. A long toll cable project involves the invest- 

 ment of such large sums of money that deferring the annual charg'e 6n 

 the cable circuit for one year will frequently be sufficient to pay for 

 and maintain a carrier system over the same period. Additional 

 carrier systems may then be added to further defer the cable if this 

 appears economical. The addition of a second system to a lead 

 usually involves some considerable line expense for transposition work, 



