inductive loading for telephone facilities 157 



Preview of Subsequent Developments 



General Outline 



For convenience in discussion and ease of understanding it has been 

 found desirable to divide the remaining subject matter of this review into 

 several parts, each covering a particular phase of the evolution of the 

 loading art, as follows: 



Part II — Loading for Long Distance Circuits. 

 Ill — ^Loading for Exchange Area Cables. 

 IV — Cable Loading Coil Cases. 



V — ^Loading for Incidental Cables in Open- Wire Lines. 

 VI — Continuous Loading. 

 VII — Extent of Use and Economic Significance. 

 VIII — Summary and Conclusion. 

 Parts II, III, IV, V, and VII are wholly concerned with coil-loading. 



In Parts II, III and V, specific coil loading systems and loading apparatus 

 developments are separately considered under headings which indicate the 

 development emphasis. In general, the individual developments are discussed 

 in chronological sequence so as to tie closely together the interrelated systems 

 and apparatus developments. The chronological procedure also applies in 

 Parts IV and VI. The dates which are given and the cross references from 

 section to section permit the reader to fit the important developments into 

 a definite time pattern. 



Although the review is primarily concerned with the evolution of loading, 

 and its contributions to the growth of the Bell System, appropriate references 

 are also included regarding other related advances in the telephone art 

 which have influenced the design and performance of the loading systems 

 and apparatus, and the extent of use. 



Loading Systems 



The changes in voice-frequency loading systems have been primarily 

 for the purpose of improving the service performance, including the trans- 

 mission of wider frequency-bands to improve intelligibility. The loading 

 changes for repeatered circuits have catered to the various special problems 

 that arose in consequence of the great increase in circuit lengths. 



The loading systems for cable circuits transmitting radio broadcasting 

 programs and those for voice-frequency and carrier-frequency impedance- 

 matching in incidental cables that occur in open-wire lines also had their 

 own individual requirements to meet the specific service needs. 



Loading Coils 



The loading coil developments substantially paralleled the loading systems 

 developments in variety and scope. In many instances, new loading coils 



