1876.] L. Sehwendler — On the General Theory of Duplex Telegraphy. 17 



Othee methods. There have heen suggested, from time to time, 

 many other methods of duplex working. On a closer examination it will, 

 however, be found that, as a general rule, they do not differ essentially from 

 the three fundamental methods treated of. I shall therefore dispense 

 with the labour of investigating these derived methods. 



In case it should be thought necessary to investigate them, no 

 difficulties ought to be met with, if only the general plan of attacking 

 duplex problems be remembered, viz., to draw the diagram of the method 

 in its most general form; develop the forces p, P, and Q ; from these 

 three forces determine the functions S and D ; find the relations which must 

 hold between the different variables (resistances and e. m. e. s.) of which 

 the system consists, in order to make S and D simultaneous minima ; con- 

 sider the question of immediate balance which determines also the best mode 

 of adjusting balance ; consider that the movement of the key must not alter 

 the complex resistance of the station to which the key belongs, i. <?., that 

 the working of the key must not affect the balance of the distant station ; 

 determine the absolute values of the different variables when balance is 

 rigidly fulfilled by considering the question of economy, i. <?., establish the 

 relations for maxima currents and maxima magnetic moments ; any vari- 

 ables which should then be left indeterminate must be fixed by secondary 

 considerations, and by certain practical conditions. 



Before comparing quantitatively the efficiency of the three fundamental 

 methods treated of, it is requred to solve two questions, viz. : — The E. M. F. 

 required for each duplex method ; the absolute size of the increments of the 

 adjustable resistance. 



[To be continued.] 



