These new construction arrangements have been worked out and 

 applied where the extensive use of carrier is sufficiently important 

 to justify them. 



As a result of these various developments, an extensive use of carrier 

 systems has been made in the Bell System plant. This is indicated 

 in Fig. 18, which shows the routes on which carrier systems are used 

 at the present time. The total circuit mileage in service provided by 

 carrier systems is about 400,000 miles, which is over 8 per cent, of the 

 total toll circuit mileage in service. 



Up to the present time, the applications' of carrier have been confined 

 to open wire, including relatively short sections of incidental cable in 

 the open-wire circuit. Further advances in the art, particularly in the 

 design of very stable amplifiers capable of amplifying simultaneously 

 a large number of carrier channels of different frequencies without 

 mutual interference and improvements in the design of electrical 

 filters to make them less expensive and more effective have opened 

 the way for broader applications of carrier. These broader applica- 

 tions include the prospective use of carrier on non-loaded cable circuits 

 with amplifiers spaced at intervals of twenty miles or less. Systems 

 are now being developed for this service by which it is expected to get 

 12 one-way channels on a single non-loaded cable pair, and with 

 cables of special construction, such as the coaxial cable, on which 

 experiments are now being made, several hundred one-way trans- 

 missions may be obtained on a single unit. 



In view of these developments under way, and further prospective 

 improvements in carrier systems applicable to open-wire circuits, it 

 is evident that this form of transmission will have in the future a 

 rapidly growing field of use in the telephone plant. 



Associated Technical Developments 



The successful operation in a practical telephone plant of the new 

 types of circuit for transmission over very long distances both in cable 

 and in open wire required, in addition to the main developments briefly 

 outlined above, the developments of a number of associated technical 

 arrangements. Some of the more important of these are briefly 

 outlined in the following paragraphs. 



Regulators 



In the long telephone circuits made possible by the use of repeaters, 

 having a number of repeaters at different points along the circuit, the 

 net transmission efficiency of the circuit is the result obtained by 

 balancing the amplification of telephone currents in the repeaters 



34 



