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BELL SYSTEM TECHNICAL JOURNAL 



Aside from manufacturing economies, the reduction in coil size is of 

 importance to the Bell System from the standpoints of plant construc- 

 tion and installation. The size reduction is particularly important in 

 instances where only a few coils are required at a given point, as is the 

 case in program circuit loading. It is now practicable to enclose as 

 many as six coils, even of the larger size employed for loading program 

 circuits, directly in the cable splice at a loading point. This dispenses 

 with the need for conventional cases, and reduces both manufacturing 

 and installation costs. In the field of small complements using the 

 inexpensive lead sheath type of case construction, it is now possible to 

 furnish as many as 100 of the small exchange area coils whereas 15 was 

 the maximum number accommodated with the superseded coil design. 

 Table III gives comparative weights and volumes of typical cases 

 provided for potting exchange area and toll type coils. 



TABLE III 



Comparative Data on Representative Cases for New 

 AND Superseded Loading Coils 



* A unit consists of one phantom and two side circuit coils. 



Figures 11 and 12 show the comparative sizes of typical steel and 

 lead sleeve type cases for the new and superseded coils. In Fig. 13, 

 the midget proportions of the latest design of case for plotting 100 

 exchange area coils are contrasted with those of the case utilized up to 

 1922 containing only 98 exchange area coils. The reduction in size 

 of cases for this type of coil is of particular importance in larger cities 

 where underground vault space is at a premium. 



Improved Inductance Coils for Filters 



The trend of development of toll transmission circuits is now very 

 definitely toward multiple channel carrier systems utilizing a much 

 wider frequency band than has heretofore been employed on wire 

 circuits.^^'^" These systems involve extensive use of selective or 

 equalizing networks at terminals and at repeater stations in order to 



