over telephone lines terminating in toll switchboards located in those 

 cities, and trunks between the subscriber switchboards and the toll 

 boards. 



As the number of subscribers and the extent of telephone service 

 increased, it became impractical and uneconomical to connect all 

 telephone subscribers in the larger cities to the same switchboard; 

 impracticable because the size of such a switchboard would be so great 

 as to make the interconnection of two lines an unwieldy and slow 

 procedure; uneconomical because of the relatively large amount of 

 telephone line which would be required to connect the more distant 

 subscribers with the central office. For these reasons means of inter- 

 connecting switchboards within a city were devised whereby a station 

 terminated on one switchboard can be connected to a station ter- 

 minated on another switchboard in the same city over a telephone line 

 or "interoffice trunk" terminating on each switchboard. The design 

 and layout of the subscriber and switchboard plant require careful 

 consideration in determining the maximum economy which can be 

 realized with the proper balance between subscriber lines and interoffice 

 trunks. 



Telephone Circuits and Cables 



At the beginnings of telephone service it was found that the iron 

 wire then used for telegraph circuits was, in many cases, not satis- 

 factory for telephony because of the losses of energy taking place in 

 the wires and the rapid diminution in the loudness of the transmitted 

 speech with the distance over which it was transmitted. At first no 

 wire was available having better electrical characteristics and at the 

 same time sufficient mechanical strength to withstand the strains it 

 was subjected to when strung on a pole line. Thomas B. Doolittle 

 of the Bell System, who was familiar with certain physical properties 

 of copper, conceived that if copper were drawn cold through a series 

 of dies, he might obtain a wire of much greater physical strength than 

 the soft annealed copper wire then used in a small way in the making 

 of electrical apparatus. In November, 1877 he arranged with a 

 manufacturer to try the process and it was so successful that in 1878 

 a quantity of hard-driiwn copper wire was placed in service in the 

 Bridgeport, Connecticut exchange. The success of this and subse- 

 quent installations showed that a wire which was electrically efficient 

 and mechanically strong had been obtained by means of which tele- 

 I)hone service could successfully be given over considerable distances. 



The numbers of wires required to serve telephone subscribers in 

 large cities led at an early date to the development of means for j)utting 



