460 



THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



Compared with the 603 coil, the new 613 (135 mh) loading coil gave a 

 material improvement in transmission, the value of which was large rela- 

 tive to the small cost-increase involved. On the other hand, the 613 coil was 

 nearly as good from the transmission standpoint as the much larger 575 

 coil and the cost per potted coil was about one-third lower. A substantially 

 similar comparison applied between the new 614 (175 mh) coil and the 

 old standard 574 coil. 



During the late 1920's the high demands for new faciUties and the plant 

 rearrangements to meet the higher cut-off loading standards combined to 

 require a somewhat larger total quantity of 613 and 614 than 612 coils. 

 The importance of the higher inductance coils dropped substantially after 

 1930, especially that for the 175 mh loading. 



19.4 618 (44 mh) and 619 {22 mh) Loading Coils 



These low-inductance coils, using the same core as the 612 coil and the 

 same types of cases, became available during 1931, primarily for use in cor- 

 recting spacing irregularities in loaded exchange area trunks. 



Table XI 

 Compressed Permalloy-Powder Core Exchange Area Loading Coils 



19.5 Subscriber-Loop Loading 



During 1933 the practice of using the 618 (44 mh) coil at M or H-spacing 

 got a good start on long subscriber loops. This new field for loading had been 

 under study for some time, and has greatly increased in importance during 

 the intervening years. In many instances, this practice makes it feasible to 

 meet the transmission limits on long loops in available 19 or 22 ga. sub- 

 scriber cables, when otherwise it would be necessary to use local battery 

 telephone sets, or install more expensive cable plant, or use relatively ex- 

 pensive telephone repeaters. Under some conditions, the 612 (88 mh) coil 

 was also used for loading long loops. 



