INDUCTIVE LOADING FOR TELEPHONE FACILITIES 459 



and in general made it feasible to provide smaller-sized loading vaults for 

 new installations. 



It is important to note that the small size and reduced cost of potted 

 612 coils led to a general use of M88 loading on non-quadded 24 ga. cables, 

 thus permitting additional cost-reductions in the cable plant. Such facilities 

 were cheaper than non-loaded 22 ga. cables, and had a greater transmission 

 range — subject in some instances to signahng restrictions. 



It is also of interest that the 612 coil was the first standard loading coil 

 sufficiently small to be placeable within loading splice-sleeves. When only 

 a few coils were required at a particular point, this method of installation 

 permitted worthwhile economies as compared with the use of conventional 

 types of loading coil cases and stub cables. 



The 612 coil remained standard for about 10 years, during which period 

 more than a million of them were manufactured. It had a much greater 

 economic impact on the fundamental design of the exchange area plant 

 than any other individual loading coil, notwithstanding the fact that the 

 present standard 88 mh loading coil, subsequently described, has already 

 been used in much larger quantities. 

 1623 Coils for HI 35 Loading 

 [624 Coils for H175 and D175 Loading 



During the development of the 603 (135 mh) iron-dust core coil described 

 in subdivision 18.2, it was fully appreciated from the transmission cost equi- 

 librium standpoint that a higher grade design would be warranted if it could 

 be obtained at a moderate increase in cost. Since this would have meant a 

 new coil-size intermediate between that of the 603 (and 602) coil and the 

 much larger 575 coil, a decision was made to use the 602 core, thereby ob- 

 taining quick savings. 



It was appreciated also that a less efficient loading coil than the 574 

 (175 mh) coil would be good enough for H175 and D175 loading, if it could 

 be obtained with a sufficiently large cost-reduction. 



These objectives were carefully considered from the cost-equilibrium 

 standpoint. It turned out that the use of a permalloy core of the same size 

 as the iron-dust core of the 602 and 603 coils would come close to an ideal 

 economic solution of the service requirements for the heavier weights of 

 exchange area loading, and accordingly the use of this size of core and coil 

 was decided upon. An important additional, immediate, economic advan- 

 tage was that the new coils could be potted in the cases originally developed 

 for the 602 and 603 coils, thus minimizing new potting developments. The 

 demand for the heavier weights of loading could be met with smaller-sized 

 complements than those frequently required for M88 loading, and conse- 

 quently no new larger sizes of cases were necessary. 



