396 ■ . Transactions. — Chemistry. 



Art. LXIV. — On Duplex Telegraphy. By Charles Lemon, 

 General Manager, New Zealand Telegraphs. 



Plates XXVI. and XXVII. 

 [Reiid "before the Wellington Philosophical Society, 2\st Noveniber, 1874.] 



My attention was drawn to the system of duplex telegi-aphy by an extract 

 from a paper read by Mr. Culley before the Society of Telegraph Engineers in 

 London, and which was published in the "Telegraph Journal." 



The system at present in use in England is based on the differential 

 principle. The system that has been adopted in New Zealand is based on the 

 Wheatstone Bridge principle. The differential principle is arrived at by 

 producing an equality of currents. The Wheatstone Bridge principle is based 

 upon an equality of tensions (now called potential). Tension or potential 

 may be considered as a term analogous to pressure, as applied to water. 



Before proceeding to describe the details on which duplex telegraphy is 

 worked in New Zealand, it will be as well perhaps to give the following 

 illustration as to the distribution of tensions in a split or derived circuit : — 

 Figure 1 (Plate XXVI.) represents two pipes running parallel to one another. 

 If at the point x a stream of water is made to enter both pipes, the pressure 

 in each will be precisely the same. Now turning to figure 2. Supposing a 

 connecting pipe, z w, be made between the two on the same level, at a point 

 in the centre of the connecting pipe, there will be no motion, for the pressure 

 on each end of the branch will be precisely the same ; for where two forces 

 meet one another, both travelling with equal and sustained velocities, at that 

 point of juncture there is no motion, the two forces having equal and opposite 

 effects, and thus neutralizing the action of each other. The case is different, 

 however, if we make the connection at a lower level in the one pipe than in 

 the other, as in figure 3. Although the pressure in each pipe still remains 

 the same, the flow from g to the point m is to a lower level, therefore the 

 pressure in the connecting pipe is all the one way, or nearly so. 



Now, the principle on which duplex telegraphy is a])i)lied in New Zealand 

 is on precisely the same conditions as shown in figure 2 ; for if we consider 

 A B and CD to be two electric circuits branching from one conductor at x and 

 joining again at y, the proportionate tension or potential is the same at every 

 equal proportional part of the resistance of the two circuits ; and at any point 

 in either circuit, provided those conditions be observed, connection between 

 the two circuits may be made, and there will be no tendency for the current 

 circulating in either circuit to pass across from one circuit to the other, for 

 although there will be a tendency for the current circulating in A B to pass to 

 C D by way of z w, still as there is the same tendency for the current circulating 

 in C D to pass to A B by way of w z, the two oppositcs neutralize one another ; 



