INDUCTIVE LOADING FOR TELEPHONE FACILITIES 191 



shape and control the transmission medium is shared by all of the transmis- 

 sion paths. Very valuable transmission advantages result from the rela- 

 tively very high velocity of transmission over the non-loaded conductors/"^ 

 One of the effects of the new carrier systems' competition was to more 

 than reverse the relative amounts of loading for new installations of 4-wire 

 and 2-wire circuits — somewhat less than ^ the total being provided for 

 4-wire circuits, during the period under consideration. The substantial cost- 

 reductions, resulting from the introduction of the 75-permeability permalloy 

 coils, of course materially limited the additional savings that could be 

 realized by further size-reduction. Notwithstanding this, and taking into 

 account also the declining demand for toll cable loading, the development of 

 the M-type loading units turned out to be a very profitable operation, in 

 terms of the reduced costs of new plant. 



11.3 SM and MF-type Molybdenum-Permalloy Core Loading Units 



These war-emergency designs owed thek existence to the necessity for 

 conserving strategic materials, especially nickel. They use half as much 

 molybdenum-permalloy as the M-type loading units. The use of a new type 

 of insulation, Formex enamel, ^p^ on the conductors in place of a combination 

 of cotton with an older type of enamel, greatly improved the winding 

 space-factor and minimized the increase in d-c resistance that necessarily 

 followed from the 50% reduction in coil size. In the frontispiece. Coil E is 

 one of these new coils. Coil D being the superseded coil. 



To minimize delays in introducing the war-emergency designs into com- 

 mercial use, which began during 1942, they were (initially) assembled in the 

 unit-containers and potted in the loading coil cases that had been designed 

 for the M-type loading units. They were initially coded as the SM-type 

 loading units. 



Subsequently, to conserve steel, entirely new unit-assembly arrangements 

 were made in new smaller-size unit containers and new smaller-size loading 

 coil cases were developed to take full advantage of the loading coil size- 

 reduction. The coils themselves were not changed but new code-designations 

 were assigned in the "MF" series, in conformity with the long established 

 practice of coding phantom loading units in their unit containers. 



The d-c resistances of the SM and MF loading units are about 25% higher 

 than those of the corresponding M-type loading units, and the hysteresis 

 effects are about 40% greater, under similar operating conditions. The 

 other core-losses are unchanged in magnitude because they do not vary as 



^°^ Published information regarding the cable carrier systems is given in Bibliography 

 References (28) and (29). 



(p) This improved wire-insulation had already found valuable uses in new exchange 

 area loading coils, as discussed in Section 22. 



