EVOLUTION OF INDUCTIVE LOADING 



1233 



In considering the two different permeability types of iron-wire and of 

 iron-dust core-materials, it is important to note that in each case the lower 

 permeability material had a much more extensive total use than the higher 

 permeability material, and that it was used in the more important facilities. 



It is of special interest from the plant-cost standpoint that nearly two- 

 thirds of the compressed molybdenum-permalloy powder core coils (up to 

 the end of 1949) are the reduced cost designs using Formex-insulated 

 conductors in their windings, this being an important factor in coil-size 

 reduction. The other molybdenum-permalloy core coils are larger-size coils 

 using a combination of textile and old type of enamel conductor-insulation. 



It is highly significant with respect to the economics of the Bell System 

 plant growth that over one-third of all voice-frequency loading coils manu- 

 factured up to the end of 1949 are of the lowest-cost types ever standardized 



Table XX 



Estimated Distribution of Accumulated Total Loading Coil Production 



Up to End of 1949 in Terms of Core Materials 



Core Material 



Fine Iron-Wire 



Compressed Powdered-Iron 



Compressed Powdered-Permalloy. . 

 Compressed Powdered Molybdenum- 

 Permalloy 



Non-Magnetic (Carrier loading) 



Approx. Period ^'^ 

 of Commercial Manu- 

 facture 



1901-1927 

 1916-1928 

 1927-1938 



1937- 

 1920- 



Note (1): For more definite dates in relation to different types of facilities, and in 

 relation to the two different permeability values of the iron-wire and iron- 

 dust materials, reference should be made to Table III (page 158). 



for general use. This total includes about 60% of the total production 

 (through 1949) of all types of exchange area loading coils. 



Loaded Circuit Mileage Estimates 



To add some substance to the significance of the production statistics on 

 voice-frequency loading, it is desirable to record some rough estimates 

 regarding the aggregate length of the cable circuits which have been loaded. 



For exchange area loading, a weighted average coil-spacing between the 

 6000 ft. and 3000 ft. values now standard can be assumed. Considering the 

 time elements in the evolution of loading practices, as discussed in Part 

 III of this review, it is reasonable to assume an average coil-spacing some- 

 what longer than the mean value of the two standard spacings, say about 

 5000 ft. On this assumption, the aggregate loaded cable-mileage which 

 corresponds with an assumed production total of 11,200,000 coils is of the 

 general order of 10,500,000 pair miles. 



