TELEPHONE SYSTEM OF THE UNITED STATES 15 



At the end of 1928 in the Bell System there were about 650 dial 

 P.B.X. installations with about 100,000 lines. The smaller sizes of 

 this equipment are designed to meet the needs of the larger residences 

 and the larger sizes are adequate for any business office and large 

 industrial plant. Typical equipment arrangements are shown in 

 Fig. 9. 



In general the private branch attendants are in the employ of the 

 subscribers having this type of equipment. It is essential that the 

 attendants be recruited and trained with the same care as central 

 office operators, and to this end, the telephone companies maintain 

 employment bureaus and training courses for the benefit of private 

 branch exchange attendants. Subscribers are encouraged to send 

 their attendants to these training courses for retraining wherever this 

 appears advisable. 



Instructors highly trained in local and toll central office operation 

 and in the best methods for handling private branch exchange work 

 constantly visit private branch exchanges in order to be of assistance 

 both to the subscribers and to the attendants in giving the best pos- 

 sible service. 



Cable Plant 



While open wires are occasionally used in limited quantity at out- 

 lying points, 96 per cent of the exchange area wire plant is in cable. 

 Of this 74 per cent is underground. 



An outstanding development is the steady increase in the number 

 of pairs of conductors which it is possible to place in a single lead 

 sheath of 6.7 cm. outside diameter. From the early use of 30 to 60 

 pair 19 gauge conductors there has been a continual increase in the 

 number of pairs and a decrease in the size of the wires until at the 

 present time 1,800 pairs of 26 gauge conductors are placed under a 

 single sheath for use in the denser areas. The significance of this 

 development is indicated in Fig. 10 which shows the year in which 

 each important step was taken and the relative cost per pair of con- 

 ductors resulting from each step in the development. 



In urban development main cables, called feeder cables, usually 

 of the maximum size, radiate from each central office through under- 

 ground conduit to the various parts of the area served. These feeder 

 cables in general are run full size for considerable distances rather 

 than being diminished at branch cable points, and the flexibility thus 

 obtained is found advantageous for conditions in this country. Each 

 main feeder cable has smaller branch cables bridged to it at intervals. 

 The main feeder cable may continue all the way through the area 



