DUPLEX TELEPHONY. 109 



generated in the other coil.* I might mention here in passing 

 that the converse is also true, namely, if the connection of one of the 

 coils be reversed, the currents generated by one coil will be opposed 

 by those of the other and no current will traverse the metallic circuit, 

 the currents will meet, however, at the subsci-ibers' lines, and, being re- 

 inforced, will proceed to ''ground" through the subscribers' telephones. 

 Advantage may be taken of this fact and two sets of transmitters 

 may be used at the centi'al ofhce or at any point in the metallic cir- 

 cuit, and they may be connected in such a manner that the sub- 

 scribers' telephones will be responsive to one and neutral to the other. 



Preferably, however, I would give the subscriber both a return 

 wire and a ground wire, and, preferably, two installations of tele- 

 phonic apparatus ; one a single installation for ordinary use, and the 

 other an installation of duplex apparatus for long-distance telephon- 

 ing on the metallic circuit. 



When repeating coils are used for charging the two branches of 

 the metallic cii'cuit, said branches are charged indirectly through said 

 repeaters. I am aware that there is nothing new in using repeating 

 coils in telephone circuits, but, so far as I am aware, they have never 

 been used as here described nor with the same object in view, namely, 

 to actuate and to be actuated by both branches of a metallic circuit 

 simultaneously. The advantage of using repeaters, as here indicated, 

 is to enable two subscribers at each end of the metallic circuit to 

 communicate simultaneously without interference, one subscriber 

 using a line connecting directly with the metallic circuit and the 

 other subscriber using a line connecting indirectly by means of the 

 two repeating coils. Hence by a combination of these two methods 

 it will be seen that either two subscribers or one subscriber and a 

 centi'al office operator at each end of a metallic circuit may com- 

 municate simultaneously without interference. 



One important advantage resulting from this arrangement remains 

 to be named, namely, the subscribei's have the advantage of two 

 conductors instead of one. This reduces the resistance of the line 

 one-half, and must necessarily very much improve the working of 

 long-distance telephone lines. This idea may be extended as follows : 

 Two metallic circuits may be connected at each end so as to form a 



* Adjustable cores are used in connection with the two induction coils for the purpose of 

 balancing, in case the two branches of the metallic circuit are not perfectly balanced. 



