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V. On Duplex Telegraphy. By Oliver Heaviside.* 



The Bridge System. 



A THEORETICAL diagram of the arrangement of the 

 conductors for duplex telegraphy by the bridge system 

 is given in fig. 1. g and g' are the receiving apparatus, which 



Ffc. 1. 



line 



may be of any kind, / and /' the batteries, and a, b, c, a', b', c f 

 are resistances. The letter attached to any branch will be used 

 to represent the resistance of that branch. The two branches 

 /and/' are preserved of constant resistance by mechanical 

 means, whether the batteries are in or out of circuit. The 

 object of the above arrangement of conductors is to enable two 

 stations, A and B, to signal each other at the same time through 

 a single line without mutual interference ; and this is accom- 

 plished by adjusting the six resistances, a, b, c, a', b r , c', so that 



and 



a :b : : c :x 

 a' :b'::c': a/, 



where x is the resistance outside station A, i. e. the resistance 

 of the line plus the resistance of station B's apparatus, and x' 

 is the resistance outside station B. When the above propor- 

 tions hold good, /and g are conjugate, likewise/' and g' ; 

 hence each station working alone produces no current in its 

 own receiving instrument ; and when they are both signalling 

 simultaneously, the currents in the receiving instruments are 

 the algebraical sums of the currents which would be separately 

 produced ; thus station A gets only B's signals, and station B 

 receives A's signals alone. It is obvious without any mathe- 

 matical demonstration that the above is true whatever may be 

 the direction and strength of the currents, provided only that 

 the two conditions ax = bc, a f x / =¥c / are satisfied, and that 

 these are the sole necessary conditions for duplex telegraphy 

 when the line has so little electrostatic capacity that the trans- 



* Communicated by the Author. 



