﻿4 L. Schwendler — On the general Theory of Duplex Telegraphy. [No. 1, 



the face of this fact it is not easy to understand why the one idea received 

 immediate and universal application, while the other, of only about 10 

 years more recent date, has met until now with universal neglect ; but on 

 closer examination it will be found that there have been perfectly compre- 

 hensible, although not all rational, influences at work. 



An enquiry into the circumstances, therefore, that have caused the 

 discovery of a system, the introduction of which must mark the second 

 great era in Telegraphy, to lie fallow for nearly twenty years is of the 

 utmost interest and cannot fail to be instructive with regard to the 

 prospects of future progress. 



From an examination of the methods originally proposed for duplex 

 working, it will be found that they do not in any way essentially differ from 

 those which may now come into actual use. The causes therefore, which 

 have prevented the introduction of the system, must be sought for external 

 to the methods. 



The first of these, we find, is that the invention was in advance of the 

 requirements of the age. Telegraph lines had already been constructed, 

 which were quite capable of carrying the given traffic and even more. 

 Further, any increase in traffic could be easily met by an increase in the 

 number of wires on the existing Telegraph posts, instead of by resorting to 

 a system, which had a complex appearance, and after all might not answer. 



However, although the above considei-ations explain the course of 

 events in certain limited instances, and up to a certain time, they do nothing 

 towards justifying the costly expedients that have been generally adopted 

 until recently in preference to introducing Duplex Telegraphy. For instance, 

 the reconstruction and multiplying of long overland lines, and especially the 

 laying of a second submarine cable when the traffic became too great for 

 one. 



It is true that the successful application of any duplex method requires 

 lines of a more constant electrical condition, receiving instruments of a 

 larger range,* and Telegraph operators of a somewhat better professional 

 education ; but, surely, these three conditions have not all at once become 

 fulfilled (since 1872) so as to make Duplex Telegraphy possible onl}- just 

 now ? No — the causes, which have delayed its introduction so long, have 

 been of a much less technical and more irrational nature. 



The mere fact of the duplex methods appearing complex prevented 

 Telegraph administrations from thinking seriously of introducing them. 

 * By the " range" of a telegraph instrument I understand the ratio of the largest 

 to the smallest force by which the instrument in question can be worked without re- 

 quiring a fresh mechanical adjustment. For instance, Siemens's beautiful relays can be 

 easily adjusted to a range of 20, i. e., they can be made to work with one cell through an 

 external resistance equal to their own resistance, and with 10 cells through no external 

 resistance, tvUhout giving the tongue a fresh adjustment. 



