of Duplex Telegraphy. 123 



station. Thus we invariably have two conditions to fulfil in 

 duplex working, independent of the particular method adopted, 

 namely :- — 



1 . The receiving -instrument of each station should not be affected 

 by its own sending. 



2. The duplex signals and single signals must be 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 sug- 

 gested fulfils these two conditions ; otherwise the method would 

 have to be rejected a priori, and could not find anyplace 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 quan- 

 titative disturbances of the two conditions (1) and (2), either if 

 we compare different methods, or the same method under differ- 

 ent 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, rt duplex signals." 



Then the first condition (1) is expressed by 



P=0, (I.) 



and the second (2) by 



P-Q=0 (II.) 



Further, if p cannot be always kept rigidly equal to zero (on 

 account of unavoidable variations in the system), we should at 

 least have 



4-=D as small as possible j .... (III.) 



