164 tHE BELL SYSTEM TECHNICAL JOUKNAL, JANUARY 1951 



had stopped. However, since exchange area circuits and suburban trunk 

 cables were seldom used for composite telegraph working, the use of 95- 

 permeability iron-wire continued standard until 1916, when compressed, 

 powdered-iron, core coils became available. 



(4) Loaded Repeatered Open-Wire Lines 



The pioneering phases of the development of better lines and better 

 loading coils for use on repeatered long-distance facilities had their first 

 commercial application in the transcontinental open-wire circuits between 

 New York and San Francisco, January 1915. The adaptation of the lines 

 to the requirements of repeater operation was secondary in importance 

 only to the development of satisfactory repeater elements, and of circuits 

 for associating the repeater elements with the line. A comprehensive ac- 

 count of all phases of the very important transcontinental telephony-de- 

 velopment project has been published in an article, "The Conquest of 

 Distance by Wire Telephony."^ Accordingly, the account in the present 

 review is limited to the loading for the line. Comprehensive information 

 regarding telephone repeaters is given in a 1919 paper by B. Gherardi and 

 F. B. Jewett.ii 



Since the lines were used for two-way transmission, a high degree of 

 impedance balance between the line and the repeater balancing-network 

 circuit was necessary in order to obtain satisfactory repeater gains. This 

 problem involved the construction of lines having a new order of regularity 

 and stability in their impedance characteristics over the working frequency- 

 band, to make feasible the design of simple types of balancing networks^^ 

 for adequate simulation of the line impedances. 



The requirements just stated involved a much greater degree of uniformity 

 in the loading coil spacing than was necessary in non-repeatered circuits, 

 and a corresponding reduction in the coil inductance deviations. This latter 

 requirement meant that the new coils must have a much greater resistance 

 to the magnetizing effects of superposed steady and transient line-currents, 

 especially since exposure to lightning surges had to be accepted as a normal 

 service experience. 



The new requirement for high magnetic stability in the loading coils was 

 met by using short air-gaps at diametrically opposite points in their toroidal- 

 type 65-permeability iron-wire cores. This construction feature also resulted 

 in a substantial reduction in (but not the elimination of) telephone trans- 

 mission distortion caused by ''telegraph flutter" phenomena, when com- 

 posite telegraph circuits were superposed on the loaded circuits. The new 

 side circuit and phantom circuit loading coils were coded 550 and 549, 

 respectively. They were a little smaller than the coils which they super- 

 seded, and had somewhat higher resistances. The resulting attenuation im- 



