122 Mr. L. Schwendler on the General Theory 



the present investigation, since they tend to show how real 

 progress in one of the youngest branches of applied science may 

 be retarded for a considerable period by nothing but prejudice 

 of the profession themselves, for whom the progress should be 

 the first essential ; and administrations will see how much the 

 advance of telegraphy will always depend on their recognizing 

 and encouraging by experiment inventions that are theoretically 

 sound and tend in the right direction. 



General Considerations. 



Before entering on the solution of the problem for any par- 

 ticular duplex method, it would be advisable once for all to 

 state definitely the nature of the general question before us. 

 This will not only save time, but the subsequent special solu- 

 tions can then also be made under a general guide ; and thus, 

 being well linked together, the whole investigation will become 

 far more lucid and concise than it otherwise would be. 



While in ordinary (single) telegraphy the signals are always 

 produced in the same way, i. e. by the signalling current arri- 

 ving through the line from the distant station, the signals in 

 duplex telegraphy may be produced in either of two ways essen- 

 tially different from each other. Namely, if the times of send- 

 ing from the two stations fall together, i. e. no current, or 

 double current, or any difference of currents is in the line, the 

 signals, so long as this state of the line exists, are produced 

 wholly or partly by the battery of the receiving- station. Sig- 

 nals produced in this way we shall call " duplex signals ;" and 

 these signals alone indicate the essential difference between 

 duplex and ordinary telegraphy. 



If, however, the moments of sending from the two stations 

 do not fall together, the signals are then produced as in ordi- 

 nary telegraphy, and may be appropriately designated " single 

 signals." 



It will be clear, then, that when the two stations are at work 

 at the same time, " duplex signals " and " single signals " must 

 necessarily follow each other in accidental succession. Nay, 

 one and the same signal produced in either station may be partly 

 a " duplex " and partly a " single ,} signal. 



To secure, therefore, regularity of working, the signals pro- 

 duced 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 requirements 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 instru- 

 ment may be entirely free to receive signals from the distant 



