TELEPHONE SYSTEM OF THE UNITED STATES 51 



present demands. Also, the toll cables provide practical immunity 

 from the effects of storms, including the sleet storms, which are a 

 hazard to open wire construction in nearly all parts of the United 

 States. 



The first long distance toll cables in the United States were placed 

 in service in 1906 between New York and Philadelphia and between 

 Chicago and Milwaukee. These cables were both placed underground 

 in multiple duct and are each about 150 kilometers long. The next 

 step in the extension of toll cables was the completion in 1914 of an 

 underground toll cable route between Boston, New York, Philadelphia 

 and Washington, a distance of 730 kilometers. Cable running west 

 from New York was completed to Chicago, a distance of 1,380 kilo- 

 meters, in 1925 and to St. Louis, a distance of 2,150 kilometers, in 

 1926. This permitted placing in service circuits entirely in cable 

 between New York and St. Louis. 



The present major toll cable routes together with the extensions 

 which it is expected to complete during the next five years are indi- 

 cated in Fig. 42. It is to be seen that in accordance with these plans 

 toll cable will, within five years, extend entirely across the continent 

 and up and down the length of both Atlantic and Pacific Coasts, will 

 extend north into Canada and south almost to Mexico. In the north- 

 eastern portion of the country where the development is the heaviest, 

 there is already a multiplicity of toll cable routes and on some of 

 these routes the rate of growth is high enough to require additional 

 cables at successive intervals of one or two years. The amount of 

 toll cable added to the network this year will be about 8,000 kilo- 

 meters and this amount is expected to be increased materially in the 

 following years. 



In the early toll cables before the extensive development of tele- 

 phone repeaters, it was necessary, in order to provide satisfactory 

 transmission, to use relatively large conductors and conductors up to 

 a maximum size of 2.6 mm. diameter (No. 10 B and S gauge), were 

 provided in the Boston-Washington cable. 



With the perfection of telephone repeaters for use with toll cable 

 circuits, the transmission limitations on the extension of toll cable 

 were removed and the economy of such circuits greatly increased by 

 making it possible to use small conductors. The longest toll cable 

 circuits at the present time are carried over conductors of 0.9 mm. 

 diameter (19 B and S gauge). For the shorter circuits each path is 

 used as a two-way circuit, while the longer circuits use separate paths 

 for transmission in opposite directions. In order to improve the trans- 

 mission characteristics, the circuits are provided with loading coils 



