456 



THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



18.3 Coils for H175 and D175 Loading 



Because of the small relative demand for new coils for these types of 

 loading, and because the 574 coil was fairly satisfactory in transmission-cost 

 relations for the relatively expensive types of facilities involved, the 574 

 coil was temporarily continued as standard for the 175 mh loading system. 

 General: Looking backwards, the classification "temporary standard" for 

 the 574 and 575 coils as used in the higher cut-off loading systems is ap- 

 propriate, and would also be appropriate for the 602 and 603 coils, by vir- 

 tue of the fact that aU of these coils were superseded as standard during 

 1927 by the new series of compressed, permalloy-powder core loading coils 

 which are described below. A brief summary of some electrical and dimen- 

 sional characteristics of the "iron-dust" core coils is given in Table X, prior 

 to undertaking the discussion of the very much more important permalloy- 



Table X 

 Iron Dust Core Loading Coils Used in Higher Cut-Off Loading Systems 



core coil development. The resistance values include 0.5 ohm for the 22 ga 

 stub cables for the loading coil cases in which the 601, 602, and 603 coils were 

 potted, and 0.2 ohm for the 19 ga. stubs used with the older coils. 



(19) Compressed Permalloy-Powder Core Exchange Area Cable 



Loading Coils 



19.1 General 



Since the general characteristics of the improved magnetic core-materiaP^ 

 and the circumstances attending its development were briefly described on 

 page 183, it is unnecessary to repeat this discussion as a part of the review of 

 the evolution of exchange area loading. It is desirable, however, to call atten- 

 tion to the fact that the exchange area loading coils were given priority over 

 the toll cabl^ ''oils in the commercial exploitation of the greatly improved 

 core-ma teri^^' for two important reasons. In the first place, the service re- 

 quirements In exchange area cables were much less complex and much less 

 severe than those in the long distance toll cables and, in consequence of the 



