496 ANNUAL EEPOET SMITHSONIAN INSTITUTION, 1915. 



extended to circuits operating on the phantom principle, and the 

 method of manufacturing cable has been completely revised, so that 

 to-day practically no large-gauge telephone cables are constructed 

 except those in which all of the wires are available for loading and 

 phantom operation. 



Incidental to these developments but of a practical importance 

 almost equal to the major developments themselves has been the 

 working out of the methods necessary properly to install and load 

 this new or so-called " duplex " type cable. 



PRESENT STATE OF THE ART WITH REGARD TO LOADING, PHANTOMING, 



AND DUPLEX CABLES. 



Coincident with the successful adaptation of loading to phantom 

 circuits and the production of a cable suitable for loaded phantom, 

 operation came a rapid application of these developments in the 

 extension and betterment of long-distance service. While these ap- 

 plications were made principally in the United States, they have also 

 been put into use in Europe and in some parts of South America, 

 notably at the Isthmus of Panama. At the present time it is stand- 

 ard practice in the United States to have all of the longer open-wire 

 lines equipped with phantom loading, to have these lines enter the 

 central offices through loaded phantom cables, and to connect the 

 principal cities with loaded phantom circuits in underground cable. 

 The most noteworthy application of phantom loading to the open- 

 wire plant is in the transcontinental telephone circuits between New 

 York, Boston, and other Atlantic seaboard cities and the Pacific coast, 

 while the most noteworthy application of loaded phantom cables is 

 between Boston and Washington, where the circuits are carried in 

 loaded underground cable pairs for a distance of approximately 

 500 miles. In England an underground loaded phantom cable 

 between London and Liverpool is now nearing completion. 



As illustrative of the extent to which loading has been applied to 

 telephone circuits in the United States within the last 10 or 12 years, 

 it might be noted that there are to-day in service approximately 

 350,000 miles of loaded inter-office trunk circuit, 34,000 miles of 

 loaded underground toll circuit, and 200,000 miles of loaded open- 

 wire lines. The average extension in the range of transmission by 

 the use of loading is from two to three times. 



TELEPHONE AMPLIFIERS. 



While the idea of telephone amplifjdng devices is almost as old 

 as the telephone art itself, it has been only within the last four or 

 five years that anything approaching successful application of this 

 idea has been made. 



