2 L. Sehwendler — On the General Theory of Duplex Telegraphy. [No. 1, 



Explanation of diagram. 



e is the E. M. F. of the line battery. 



/3 its internal resistance. 



E is the E. M. F. of the compensation battery. 



a its internal resistance. 



K is a constant resistance key. Dr. G-intl used an ordinary key, which, 

 it will be obvious, must result in a failure. 



h is an ordinary key ; both keys, in the same station, are worked simul- 

 taneously, i. e., contacts 4 and 5 are closed and broken at one and 

 the same time. 



d,f, and w are certain resistances. 



a is the one coil of the differential instrument which is connected up in 

 the line circuit. 



o is the other coil of the differential instrument which is connected up 

 in the compensation circuit. By a and b shall be also designated 

 the resistances of these two coils. 



The coils a and b with their batteries e and E respectively are arranged 

 in such a manner that they have opposite magnetic effects with respect to 

 the same magnetic pole. The two circuits in each station (the line circuit, 

 and the compensation circuit) are insulated from each other. All the other 

 terms, as L, L', L", &c, shall have the same physical meaning as before. 



The compensation method has two principal defects which the two 

 preceding methods do not possess. 



Firstly. The success of working a line duplice by the compensation 

 method will clearly depend on the possibility of being able to close and open 

 simultaneously two different contacts (4 and 5). The mechanical difficulty 

 of doing so sufficiently accurately was pointed out by Dr. Werner Siemens, 

 and in fact constitutes one of the reasons which led him to propose the 

 differential method. 



Secondly. The balance in each station may be disturbed directly by a 

 variation of the electrical condition (internal resistance and e. m. f.) of the 

 two batteries (E and e) employed. 



In the preceding two methods the variation of the internal resistance 

 of the signalling battery can only be felt indirectly by affecting the balance 

 of the distant station; while the variation of e. m. e. has no effect at all. 

 Hence a given variation in the battery or batteries must necessarily produce 

 a greater disturbance of balance in the compensation method than in the two 

 preceding ones. We know that even so-called constant galvanic batteries, 

 doing work, alter their electrical conditions perceptibly, especially their 

 internal resistance, and consequently this defect weighs most decidedly 

 against the compensation method. In all other respects the compensation 



