6 L. Schwendler — On the General Theory of Duplex Telegraphy. [No. 1, 



It is scarcely needed to point out that to adjust balance by altering the 

 line current, either by varying the resistance or the E. M. i\* or both of the 

 line circuit, must be rejected once for all, because such an adjustment of 

 balance in the one station could never take place without disturbing the 

 balance of the other station ; or in other words the required immediate bal- 

 ance could not be fulfilled. 



b. Re-adjustment of balance by moving the coils or armatures. 



If we suppose both the coils or their armatures simultaneously mov- 

 able in the same direction, then clearly this mode of adjustment contains 

 not only the required immediate balance, but in addition represents also a 

 very rapid and entirely continuous action. For this reason it is apparently 

 preferable to the first method, where the adjustment can only be carried on 

 in one branch by varying d in increments, f Which of the two methods, 

 however, is to be chosen finally, depends on other considerations which will 



* Alteration of e. m. p. of a galvanic battery cannot be achieved without altering 

 its internal resistance. Hence varying e, would also involve a variation of /3, and in 

 order to keep / = w + 13, it would become necessary to alter w simultaneously with e, 

 i. e., iv would have to be increased when e decreases and vice versa. This method 

 being rough, would therefore be also inconvenient. 



f It has been suggested to adjust balance by a continuous variation of resistance, as 

 for instance by moving a contact point along a thin platinum wire in the same manner 

 as Dr. "Wr. Siemens has done in his bridge employed for comparing accurately com- 

 paratively small resistances. It is, however, scarcely necessary to point out that such a 

 method, if applied for Duplex "Working, must result in a failure, at all events so long as 

 electro-magnetic instruments are used for producing the signals. For in such a case, 

 the resistance of any branch, no matter what special Duplex method may be employed, 

 must bear a certain ratio to the given resistance of the line, in order to get the signals 

 with sufficient force. This ratio, as my investigations have shewn, is by no means a 

 small one, and hence the resistances of all branches, even for a short line, cannot be made 

 small. Therefore the platinum wire, constituting part of one or two branches of the 

 Duplex method employed, must also offer a considerable resistance, i. e., must be of great 

 length. Hence to alter such a large resistance continuously and perceptibly, as is in- 

 dicated by the balance disturbance, must evidently involve a considerable movement of 

 the contact point, which, even choosing the thinnest possible wire, and the shortest Tele- 

 graph line, becomes already for the daily variation so large as to make its application 

 impossible. Unless another material of much higher specific resistance than Platinum 

 wire can be found which, at the same time, allows of the sliding contact being made 

 securely, the adjustment of balance by a continuous variation of resistances must be 

 dispensed with. Such a material does not appear to exist. I thought of acting on 

 Phillips's suggestion to use pencil-marks for the adjustable resistance, and although I 

 found that pencil-resistances can be adjusted very accurately, and can be enclosed in a 

 very small space, and that they keep sufficiently constant, it is difficult, if not impossible 

 to alter them by a sliding contact. The " Uebergangs-tviderstand" is too variable and too 

 great. Besides, if the contact is made with sufficient pressure, its sliding along alters the 

 thickness of the pencil mark, and hence the resistances become inconstant and uncertain. 



