INDUCTIVE LOADING FOR TELEPHONE FACILITIES 469 



having the same dimensions as the 125-permeability cores of the 632 coils. 

 The small over-all dimensions of the coils made it practical to mount them 

 within the "connectors" that terminated each quarter mile length of spiral- 

 four cable, without requiring the connectors to be appreciably larger than 

 otherwise would have been necessary. Thus, in effect, the loading was built 

 into the cables at the factory, thereby simplifying installation. Another re- 

 markable feature of the loading was that it had a cut-off frequency of about 

 22 kc and provided satisfactory transmission for cable carrier systems using 

 a frequency-band extending to 12 kc. One indication of the importance of 

 the coil under discussion was that nearly two million of them were manu- 

 factured for the United States Signal Corps before VJ day. 



22.4 Impact of Strategic Material Scarcities on Loading Coil Design 



Before the redesign of other loading coils could be undertaken to take 

 advantage of the space-saving possibilities inherent in the use of Formex- 

 enamel insulation, a new design factor suddenly became controlHng. Nickel 

 had become a strategic war material, and accordingly severe restrictions 

 were placed upon its use, including all magnetic alloys in which nickel was 

 a constituent. Molybdenum-permalloy was in this category. 



This made it necessary to redesign the toll cable phantom loading units, 

 as mentioned in the description of the "SM" type loading units (Section 

 11.3), the high-inductance exchange area loading coils, and certain non- 

 phantom type toll cable loading coils used principally for "order-wire" 

 circuits in coaxial cables. 



As the new low-inductance exchange area coils (632, 638, 639), previously 

 described, used only a very small amount of nickel (about 0.9 oz. per coil), 

 no further worth-while reductions in the core size could be obtained with- 

 out objectionable reactions on transmission, and without undertaking ex- 

 tensive development work that would have interfered objectionably with 

 much more important war jobs. Consequently, the new 632, 638, and 639 

 coils were continued as standard designs. Large quantities were used during 

 the war, and very much larger quantities since VJ day. 



22.5 643 {135 mh), 644 {175 mh), and 645 {250 mh) Exchange Area Loading 

 Coils 



Since the standard 623, 624, and 625 coils, previously described, used 

 about four times as much nickel in their cores as the 622 (and 632) loading 

 coils, their redesign became an important factor in the new development 

 program to conserve nickel. 



A relatively simple solution for this specific problem was worked out, 

 namely to use Formex-insulated conductors on the cores developed for the 

 622 series of coils. This saved three-quarters of the nickel used in the 



