240 BELL SYSTEM TECHNICAL JOURNAL 



auguration of commercial transcontinental telephone service over the 

 New York-San Francisco line in January, 1915, marked the first com- 

 mercial application of these general improvements in regularity of 

 line construction, including the use of an improved type of loading coil. 



In the extensive field work which was done in preparing for trans- 

 continental telephone service, it was found that the inductance values 

 of a considerable percentage of the open-wire loading coils then in use 

 (Nos. 511 and 512 types. Table VII) had changed appreciably from 

 the nominal values to which they were adjusted at the factory prior 

 to shipment, and that these changes were due to core magnetization 

 caused by abnormal currents induced by lightning discharges. In 

 some cases abnormal currents induced by power transmission lines 

 or electric railway distribution systems were responsible for the loading 

 coil magnetization. 



The inductance changes were not sufticiently large to have serious 

 reactions on transmission over non-repeatered circuits. Although 

 individual coils varied in inductance from time to time, the general 

 average of groups of coils was fairly constant. The eff'ects of these 

 individual variations on the impedance of the line were, however, 

 too large to permit satisfactory operation with telephone repeaters. 

 .Some experiments made with improved lightning arresters, in an 

 effort to reduce the coil magnetization trouble, were unsuccessful. 



The solution of the problem of repeatering loaded open-wire circuits 

 required the development of loading coils which would be stable 

 magnetically when subjected to extreme conditions of magnetizing 

 current in the windings. The requirement was laid down for these 

 coils that the inductance to speech currents should not be affected 

 more than about 2 per cent, when a magnetizing current of two 

 amperes was passed through either line winding. In view of the fact 

 that the extreme residual magnetizing effect of this current on the 

 No. 511 and No. 512 loading coils was approximately 30 per cent., it will 

 be appreciated that this imposed a very severe stability requirement. 



The design adopted involved the use of air-gaps in the cores of the 

 iron wire core loading coils. ^^ Two air-gaps were employed at oppo- 

 site points in the cores and suitable clamping means were provided 

 to hold the coil halves in proper alinement. The use of only two 

 air-gaps in the cores of the phantom loading coil brought in unbalance 

 tendencies not present in older designs, which were corrected by special 

 refinements in the design. 



The use of a magnetic circuit having "ends," while effective for 

 producing self-demagnetization, brought in troublesome magnetic 



" U. S. Patents Nos. 1,289,941 and 1,433,305— Shaw and Fondiller. 



