i.o.inixc lOR I ii.i.i'iioM- cih'cr/rs 



27r> 



Loadhiii^ Coils luid Cases. As prcxioiisK noted, the lirst imijorlaiit 

 change in ilu- coils used for exchange area loading from the early 

 95-permeabilil\- wire core type was the stibslitulion of compressed 

 powdered iron in i)lace of wire for the cores. Initially, only coils 

 having jjowdered iron cores with a permeability of (K) were designed, 

 as this \altie corresponds to the etTective value of the cores displaced. 

 More recently, in order to better fit in with the reciuirements of the 

 new cut-ofT freciuency standard, coils using 35-permeability powdered 

 iron cores have been developed. In Table XMI are listed data 

 for the coils now used in exchange area loading. 



TABLE XVII 



Coils for Loading Exchange A rea Cables 



Effective resistance values are for a line current of 0.001 ampere. 



The standardization of the small size Nos. 602 and 603 loading 

 coils has made it possible to design containing cases and assembly 

 methods which permit much larger numbers of coils to be enclosed 

 in cases conforming to the dimensional limitations set by existing 

 vault conditions. A series of cases having capacities up to 300 coils 

 has now been developed. The use of these large potting comple- 

 ments will be of considerable value in reducing the space congestion 

 encountered in the "downtown" sections of the larger metropolitan 

 areas. 



In the 300-coil case, a total of 1,200 soldered joints are required 

 to connect the coil terminals to the stub cable conductors. It was 

 accordingly very important that the assembly method should involve 

 a minimum liability to open circuits, crosses, or grounds. To ac- 

 complish this, a method was devised whereby the various spindles of 

 coils were assembled to a skeleton frame to which the cable stub con- 

 taining the 600 terminal pairs is also attached. All splices to the 

 outgoing conductors are made immediately adjacent to the individual 



