DEVELOPMENTS IN TELEPHONY AND TELEGRAPHY JEWETT. 495 



To be effective, loading coils must provide the required inductance, 

 without at the same time too greatly increasing the effective resist- 

 ance of the line. With the materials at first available it was feared 

 that the only feasible way of obtaining the desired transmission 

 results would be through the employment of air-core coils. Coils of 

 this type were actually constructed and installed during some of 

 the early experiments. Wliile fairly efficient in the matter of 

 the ratio of inductance to effective resistance, these coils were abnor- 

 mally large and had the veiy serious defect of producing a large 

 stray mag-netic field, so that it was impossible to locate more than a 

 few coils in close proximity to one another without encountering 

 serious cross interference. 



This early work showed the absolute necessity of producing a mag- 

 netic core type of inductance coil. The solution of this problem led 

 ultimately to the development of the fine iron wire toroidal core type 

 of loading coil which is now standard. 



By 1906 suitable coils had been developed for loading both cable 

 and open-wire circuits of the ordinary metallic type. In the cable 

 plant loading was common for all of the longer interoffice trunks in 

 cities and loaded underground toll cables between cities were begin- 

 ning to be installed. In the open-wire plant loading was common 

 for everything except the larger sizes of copper wire which were 

 employed in the very long distance services. The loading of such 

 wires had not as yet proved feasible, due to the fact that with the 

 construction then standard the reduction in transmission efficiency in 

 times of wet weather was so great that the loaded circuit was at times 

 actually poorer than a corresponding nonloaded circuit. 



At the time in question (1906-T) no progi'ess had been made in the 

 application of loading to circuits operating on the phantom prin- 

 ciple. In laying out the plant the telephone engineer was con- 

 sequently confronted with the necessity of choosing either to avail 

 himself of the benefits of better or more extended transmission 

 through loading or of greater circuit facilities through phantoming — 

 he could not obtain both. 



In this connection it is interesting to note that whether the choice 

 was loading or phantoming, the successful operation of plant neces- 

 sitated a very much higher degree of line construction and mainte- 

 nance than had hitherto been deemed necessary. 



DEVELOPMENT OF PHANTOM LOADING AND DUPLEX CABLE. 



The engineers of the Bell system were fully alive to the disad- 

 vantages of the conditions just described and at once commenced 

 developments looking to their elimination. The result has been that 

 during the past eight or nine years the problem of loading large- 

 gauge open-wire circuits has been solved, the use of loading has been 



