INDUCTIVE LOADING FOR TELEPHONE FACILITIES 465 



tion of the core winding-space. Although the percentage cost-reduction was 

 not large, the aggregate savings were large in consequence of the substantial 

 amount of new loading required. The reductions in potting costs of the new 

 smaller-size loading coil cases, and the savings in installation costs were 

 important factors in the total savings. 



The new 44 mh and 22 mh loading coils. Code 628 and 629 respectively, 

 used the cores designed for the 622 coil. They were substantially equivalent 

 in transmission performance to the 618 and 619 coils. The new 628 (44 mh) 

 coil became quite important in subscriber-loop loading. Over the years 

 during which they remained standard, the average annual production was 

 about one-fourth that of the 622 coil. Relatively very few 629 coils were 

 used. 



21.3 623 (135 mh), 624 {175 mh), and 625 {250 mh) Loading Coils 



The new 135 mh coil had about the same size and efficiency relations to 

 the 613 coil, as those that existed between the new and old 88 mh loading 

 coils (612 and 622). The entirely new size of core which was made available 

 for it was also used in the relatively unimportant, new higher-inductance 

 coils. The winding machine developed for the 612 coil was used in winding 

 the coils under discussion. 



The over-all dimensions of the new coils were intermediate between those 

 of Coils H and G in the headpiece, being closer to H than to G. The expected 

 demand for the 623, 624, and 625 coils was not large enough to warrant the 

 development of an entirely new series of loading coil cases especially for 

 these coils. As these non-phantom coils were being developed concurrently 

 with the molybdenum-permalloy core side circuit and phantom circuit coils 

 for toll cables, i.e., the M-type loading units described in Section 11.2, 

 arrangements were made for potting them in the new cases that were de- 

 veloped for potting the loading units. However, different assembly arrange- 

 ments and stub cables were required. Since the non-phantom coils were 

 only about 20% smaller than the coil components of the loading units, this 

 potting procedure was not unduly expensive for the non-phantom coils. 



The percentage savings resulting from the development of the 623, 624, 

 and 625 coils was larger than that yielded by the 622, 628, and 629 coils but 

 the aggregate savings were much smaller in consequence of the much lower 

 demand for the higher-inductance coils. 



(22) Redesign of Exchange Area Loading Coils to Take Advantage 

 OF Use of Formex Insulation on Winding Conductors 



22.1 General 



During the late 1930's a greatly improved type of enamel insulation 

 (developed by the General Electric Co.) known as "Formex" became com- 



