DEVELOPMENTS IN TELEPHONY AND TELEGRAPHY JEWETT. 491 



by the application of sound engineering principles, had indicated 

 certain things as being fundamental to the proper extension of com- 

 mercial telephony. The trend of further development and the cor- 

 rect methods to be followed were also becoming clear. 



Among other things, the necessity for usmg metallic circuits had 

 been proven beyond peradventure. So, too, the employment of the 

 relatively high-priced, hard-drawn copper line wire had been shown 

 to be economically cheaper and more satisfactory, all things con- 

 sidered, than the inexpensive iron and steel wire originally employed 

 for both telephone and telegraph circuits. Moreover, the fallacy of 

 building expensive long-distance, open-wire pole lines and terminat- 

 ing them in the cities in small wire cables was beginning to be fully 

 appreciated, and the engineering of the wire plant was already being 

 done on a scientific basis. This basis was that the wire plant as a 

 whole should be in cost equilibrium when considered from a trans- 

 mission standpoint. In a plant where such cost equilibrium exists, 

 the increased annual charge required to give a fixed increment of im- 

 provement in transmission is the same, irrespective of the part of the 

 plant in which the improvement is made. The full appreciation of 

 this fundamental requirement and its extended application has prob- 

 ably done more than any one thing to eliminate gross variations in 

 the grades of transmission furnished in different localities and greatly 

 to reduce plant costs. 



Fifteen years ago the possibility of improving the transmission 

 efficiency of telephone circuits by the periodic introduction of loading 

 coils was not commercially known. Little or no progress had as yet 

 been made in the art of securing a third or " phantom " metallic 

 circuit from two ordinary metallic circuits. Amplifying devices, 

 which were among the earliest dreams of the telephone inventor 

 and engineer, were still in a crude state of undeveloped laboratory 

 equipment. Fifteen years ago the telephone engineer, except in a 

 few localities, had had relatively little experience in the problem 

 of operating in close proximity to electric power circuits of extra 

 high potential. To be sure, the introduction of street railways 

 and low-tension lighting and power circuits had already brought 

 with them the necessity for radical changes in the telephone art. 

 But the single-phase railway and the high-potential transmission 

 circuit employing hundreds of thousands of volts did not then exist. 



In the matter of substation apparatus — ^that is, transmitters, re- 

 ceivers, and associated devices — the telephone art had become some- 

 what stabilized. The bipolar type of receiver was in general use, 

 as was also some form of multicontact microphone. In America, 

 where the ultimate scope of telephony was recognized to include a 

 universal long-distance service as well as purely local service, the 

 solid back, gi\anular button type of carbon transmitter had come to 



