INDUCTIVE LOADING FOR TELEPHONE FACILITIES 449 



of applying loading to the entire length of the circuit. In effect the partially 

 loaded circuit is a tandem combination of loaded and non-loaded circuits, 

 with the loaded part preferably located near the center. The purpose is to 

 reduce plant cost by restricting the use of loading in individual circuits to 

 about the minimum amount that would be necessary to meet the trans- 

 mission Umits set up as objectives in plant design. In these practices, cer- 

 tain minimum limits regarding the number of loads per circuit were worked 

 to on the basis of engineering experience, different limits being appUed in 

 different operating areas. 



15.3 Compressed Iron-Powder Core Loading Coils 



The first important change in loading coil standards for exchange area 

 loading occurred during 1916, immediately following the successful develop- 

 ment of the compressed annealed, powdered-iron core-materiaF described 

 on pages 167-170. In general, the coils that used this new core-material were 

 much better suited to the requirements of exchange facilities than to those 

 of toll cables. The old standard 95-permeabiJity iron-wire core coils, Codes 

 506, 507, and 508, were superseded as standards for new plant by the new 

 Nos. 573, 575, and 574 loading coils, respectively. The new coils had closely 

 similar over-all dimensions to those of the superseded coils, and were sub- 

 stantially equivalent, or slightly better, with respect to steady-state trans- 

 mission properties. They were greatly superior with respect to their resistance 

 to permanent or quasi-permanent magnetization by strong currents that 

 might flow through their windings in consequence of accidental grounds on 

 d-c signahng circuits, or from other external causes, including power-line 

 crosses and lightning surges. 



For a period of several years, the loading practices with the new coils 

 followed those which had evolved in the use of the older coils. 



(16) Development of Cheaper Cables for Exchange Areas and 

 Standardization of New Loading Systems for Them 



16.1 The New Cables 



During the early 1920's new, cheaper types of non-quadded cable began 

 to be used extensively in the exchange area plant. These resulted from the 

 continuing development work to reduce plant costs. By including design 

 features that made them suitable from the crosstalk standpoint for the 

 application of loading, the economies inherent in the use of loading sub- 

 stantially augmented the large economies that directly resulted from the 

 lower costs of the cables. These design improvements included the staggered 

 pair-twist construction and other features previously appUed to the 0.066 



