Some Very Long Telephone Circuits of the Bell System

E C E N T papers 1 have discussed at length the use of toll cables for the handling of certain long distance traffic. These cables, which are being used in areas of dense traffic, have been made possible by m a n y developments in cable design, repeaters and loading coils. Coincident with these developments have gone others which are finding their application in the extensive establishment of improved open wire circuits for use over very long distances. T h e purpose of the present paper is to discuss some of the considerations involved in the overall design and maintenance of these very long open wire circuits. These circuits are often referred to as " b a c k b o n e " circuits and supply a network of t r u n k lines for the entire Bell System. The most i m p o r t a n t of these routes are shown in Fig. 1. The first transcontinental line was completed in the summer of 1914 and early in the following year three transcontinental telephone circuits were placed in commercial service. These circuits were constructed of copper wire 165 mils in diameter loaded with 250 millihenry coils at intervals of 7.88 miles and had telephone repeaters located at points about 500 miles apart. The opening of these first circuits, while marking a most i m p o r t a n t stage in the progress of long distance telephony, has been followed by m a n y developments which have made possible increased overall transmission efficiency and improved quality. A discussion of these developments is given in a paper on "Telephone Transmission Over L o n g Distances," by H.