470 THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



623 series of coils. By using the 622 core instead of the 632 core, a larger 

 winding-space became available for the same savings in nickel, and a lower 

 winding resistance was obtained. New loading coil cases were not required, 

 since the redesigned coils could be potted in the cases developed for the 622 

 series of coils. 



The use of the much smaller cores necessarily resulted in resistance val- 

 ues that were substantially higher than those of the 623 series of coils, 

 notwithstanding the improved winding-space efficiency of the Formex- 

 enamel conductor insulation. The increments in the d-c resistance , (relative 

 to the 623-coil series) were a little over 60%. The effective resistances at 

 1 kc were approximately 50% greater. The attenuation impairments that 

 resulted from the increases in resistance were in the general range 0.01 to 

 0.02 db/mi at 1000 cycles, depending upon the type of cable and weight of 

 loading, and were considered to be tolerable for war-emergency designs. 



22.6 641 (44 mh) and 642 [88 mh) Non-Phantom Toll Cable Coils 



These are briefly mentioned here because of their general similarity, ex- 

 cept as regards inductance and resistance, to the 643, 644, and 645 exchange 

 area coils described in the preceding paragraphs. They also make use of 

 cores developed for the 622 coils and utihze Formex-insulated conductors. 



These coils are replacement "nickel-saving" designs for pre-war standard, 

 "toll-grade" non-phantom type of cable loading coils, which were of about 

 the same size as the side-circuit loading coils used in the M-type loading 

 units. During the war the new coils had a moderate use as substitutes for 

 SM-type loading units on toll cables, thereby saving additional amounts 

 of nickel. The 641 (44 mh) coil has about the same resistance characteristics 

 as the side circuits of the SM-type 44-25 mh phantom group loading units. 

 A similar general relation exists between the 642 (88 mh) coil and the side- 

 circuit of the SM-type 88-50 mh loading units. 



The present principal field of use for the 641 and 642 coils is on 4-wire 

 type and 2-wire type "order-wire" circuits in coaxial cables for use in the 

 operation and maintenance of coaxial cable systems. Some of these order- 

 wire circuits are as long as or longer than the longest loaded commercial 

 message circuits used prior to the general introduction of cable carrier sys- 

 tems into the toll cable plant. The 643 coil also is occasionally used on short- 

 haul order-wire circuits in coaxial cable systems. 



22.7 651 (44 mh) Coil for Subscriber-Loop Loading 



This was a post-war development looking towards the reduction in cost 

 of subscriber-loop loading. 



During the war, the design of a radically new type of automatic winding 

 machine made it feasible to apply fine-wire, high-inductance windings on a 



