DUPLKX TKLEPHOXY. 



107 



directions ajul consequently neutralized each other. Hence while 

 two subscribers, one at each end of the metallic circuit, were using 

 the line, nothing would be heard in the central office telephones. 



The first problem having l)een .solved, I was encouraged to attempt 

 the solution of the .second, namely, how to render the subscribers' tele- 

 jihoues neutral to the central office transmitters. This was found to be 

 a difficult problem. It was obvious that if the transmitters were placed 

 in either branch of the metallic circuit, the electrical currents gen- 

 erated by said transmitters would divide at the junction of the 

 subscribers' lines and a portion of said currents would escajie to ground 

 through the subscribers' telephones. Without stopping to enumerate 

 the different expedients resorted to, I finally saw that the only solu- 

 tion to the i)roblem was to arrange the central office transmitters .so 

 that both branches of the metallic circuit would be charged equally, 

 simultaneously, and in opposite directions. By chai'ging one wire 

 positively and the other negatively at the same time, I saw that the 

 electrical equilibrium between the two wires could be disturbed with- 

 out affecting the equilibrium of the subscribers' lines. Fortunately 

 after many and various experiments the means for accomplishing 

 this were discovered and the problem was finally solved. 



T also discovered that this can be accomplished in several ways. I 

 will call your attention to but two of these. The first of these 

 methods is to use a ti'ansmitter with two induction coils, the primary 

 coils being connected either in series or in multiple arc, and the 



Fig. 1. 

 In Fif,'. 1 A. A. and B.B. are two branches of a metallic circuit, and C.C. are two subscribers' 

 lines attached thereto. D. and D', subscribers' transmitters, and E.E. are receiving- telephones 

 at two central offices at each end of the metallic circuit respectively. The telephones E.E. 

 have two coils, a and b, the former connected with wire A. and the latter with wire Ji. These 

 coils are connected diflferentially, and the telephones are neutral to the snbscribers' trans- 

 mitters. 



