﻿1874.] L. Scliwcndler — On tlie general Theory of Duplex Telegraphy. 7 



To secure, therefore, regularity of working, the signals produced in 

 either way should be invariably of equal strength. 



Further, as in Duplex Telegraphy the receiving instruments must be 

 always permanently connected up with the line, it is one of the first require- 

 ments that the out-going or sent current from any station should in itself 

 have no effect whatever on the receiving instrument of that station, in order 

 that the instrument may be entirely free to receive signals from the distant 

 station. Thus we invariably have two conditions to fulfil in duplex working, 

 independent of the particular method adopted, namely : — 



1. The receiving instrutnent of each station should not le affected hy 

 its own sending. 



2. The duplex signals and single signals must he of equal strength. 



If these two conditions, which are necessary and sufficient, could be 

 always fulfilled, Duplex Telegraphy would be entirely on a par with single 

 Telegraphy, for the sending would not only not interfere with the receiving — 

 the more important condition of the two— but the received signals would 

 also be constant in strength, and, therefore, frequent adjustment of the 

 receiving instrument would be no more required than in single Telegraphy. 



Theoretically of course every duplex method hitherto suggested fulfils 

 these two conditions, otherwise the method would have to be rejected a 

 priori and could not find any place in this paper. 



Practically, however, the different methods may behave very differently 

 with respect to the fulfilment of these two conditions, nay, even one and 

 the same method is sure to give quite different results in this respect by 

 only altering the magnitude of the resistances of which the arrangement 

 consists. For in practice variations, especially in virtue of the line having 

 by no means a constant electrical condition, are necessarily going on. These 

 unavoidable variations it is clear may cause very different quantitative dis- 

 turbances of the two conditions (1) and (2) either if we compare different 

 methods, or the same method under different resistance arrangements. 



To make the foregoing clear, we will designate : — 



By p the force which acts on the receiving instrument on account 

 of not being able to fulfil the first condition absolutely ; 



By P the force which acts on the same instrument, when the distant 

 station is sending alone, i. e., " single signals ;" 



And by Q the force which acts on the same instrument, when both 

 stations are sending simultaneously, i. e., " duplex signals." 



Then the first condition (1) is expressed by : — 



p = (I) 



and the second (2) by 



P _ Q = (II) 



