198 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



The satisfactory completion of the system's trial-installation tests in 1929 

 resulted in a large amount of B22 loading being installed during 1930 and 

 1931 on 16-gauge pairs in cables that had been installed in advance of the 

 facility development work, as previously mentioned, and in new cables. 



During 1936, compressed, molybdenum-permalloy powder-core program 

 loading coils became available for use in place of the permalloy-core coils 

 above described. The new coils were smaller than their predecessors, and 

 were substantially as good or better in the lower half of the working frequency 

 band. At the high frequencies they were better than the permalloy-core 

 coils, because of the lower eddy-current losses previously mentioned. An 



Fig._ 15 — Molybdenum -permalloy core program circuit loading coil potted for installa- 

 tion within cable splice sleeves. The canvas bag shown in the picture is used to prevent 

 the metal shielding-container of the coil from damaging the insulation of nearby con- 

 ductors in the cable sphce. The fabric strips are used to tie the coil case to the cable core 

 at the splice. 



important factor in the choice of coil size was to make the coil suitable for 

 installation within the loading splice-sleeves so as to reduce potting and 

 installation costs. This practice became quite general, especially at the "B" 

 loading points intermediate between the "H" loading points where phantom 

 loading units were installed on telephone message circuits in the same cable. 

 In these "splice installations," the per-coil savings in potting and installation 

 cost were large relative to the direct savings in loading coil costs resulting 

 from size reduction. In order to realize these substantial combined savings 

 as quickly as possible, the new program coils were given priority in the 

 commercial exploitation of the molybdenum-permalloy core-material. 

 As an indication of the commercial importance of the program circuit 



