of Duplex Telegraphy . 



459 



first problem for the diffe?°ential method, which in practical im- 

 portance ranges second to the bridge method. 



II. Differential Method*. 



This arrangement for duplex working is based on the well- 

 known method of comparing electrical resistances " differential 

 method f and fig. 2 gives the general diagram when this method 

 is applied for duplex working. 



HTg.2 





T 2 



Sill 



E, electromotive force of the signalling; battery. 



/3, internal resistance of the signalling battery. 



k, a constant-resistance key. 



a and b, the coils of the receiving- instrument. These coils, for any sent 

 current, have opposite magnetic effects with respect to any given magnetic 

 pole external to the coils ; while for an) r received current these coils add 

 their effects with respect to that same magnetic pole. By a and b shall 

 also be designated the resistances of the coils. 



d, w,f, and h are certain resistances, the necessity of which will become 

 clear hereafter. 



i, the resistance of the resultant fault of the line, acting at a distance 

 V from station I., and at a distance I" from station II. (both V and I" ex- 

 pressed in resistances, so that Z / +Z"=L equal the " real conduction-resist- 

 ance " of the line). 



The other terms, viz. L', L", p', p", c', c", &c, which will necessarily be 

 of frequent occurrence also in this paper, will bear the same physical mean- 

 ing as before. 



The practical inferiority of the differential method, when com- 

 pared with the bridge method, it will be clear at once, is that 

 specially constructed receiving-instruments on the differential 



* The differential method was originally invented, as stated before, by 

 Mr. Frischen and by Messrs. Siemens and Halske. A particular case of 

 this method was patented by them in England in 1854. 



