EVOLUTION OF INDUCTIVE LOADING 122'>> 



then accumulated total demand. In general, the loading coils were manu- 

 factured to meet specific customers' orders; manufacture for merchandise 

 stock in anticipation of future orders was seldom undertaken, except during 

 periods of extraordinarily high, sustained, demand. On this basis practically 

 all of the coils that were manufactured were installed in the telephone plant. 

 The production statistics of course include a considerable number of 

 coils w^hich were installed shortly after manufacture and which were taken 

 out of service many years later to facilitate the use of improved transmission 

 systems that required different types of coils, or to permit the use of carrier 

 systems on the unloaded toll cable circuits. In general, complete potting 

 complements were not taken out of service in preparation for carrier sys- 

 tems operation; i.e., a large fraction of the disconnected loading coils remain 

 in the cases in which they w^ere originally potted and installed, and the 

 other coils in the same cases are still in service. It is important to remember 

 that the displaced loading coils played an important part in the improve- 

 ment and growth of telephone service in their own period of commercial 

 use. The unavailability of statistics regarding displaced loading makes it 

 impossible to supply accurate information regarding the total number of 

 loading coils now being used for regular telephone service. It seems probable, 

 however, that about 80% or more of all the toll cable coils that have been 

 manufactured are in service, or installed in circuits which will be used as 

 soon as trafiic growth requires them. The corresponding percentage figure 

 for exchange area coils is probably higher. The number of loading coils 

 taken out of service because of incipient defects that were not detected in 

 the factory inspection tests, or which became unserviceable in consequence 

 of service injuries, or which have been junked because of obsolescence, is a 

 very small fraction of the total number of coils that have been manufactured 

 lor Bell System use 



General Production Statistics, Voice-Frequency Cable Loading 

 Total Production 



The grand total production figure (up to the end of 1949) for all types 

 of voice-frequency loading coils for Bell System use is of the order of 20.7 

 million. Approximately 54% of this total (about 11,270,000 coils) are non- 

 phantom type coils, used almost entirely on exchange area non-quadded 

 cables. Nearly 9,500,000 coils are side circuit or phantom loading coils 

 used on quadded toll and toll entrance cables. Approximately three-quar- 

 ters of the grand total have been manufactured during the last two decades. 



The greatly varying rates in the growth of loading coil production are 

 shown, (a) in terms of accumulated total production through 1949 in 



