Mr. 0. Heaviside on Duplex Telegraphy, 39 



Ci as small as possible, c must be as large as possible ; and this 

 will occasion, a great loss of working current. This, however, 

 will be of little consequence with the delicate instruments 

 used on cables, if ever duplex telegraphy is successful on them. 



The Differential System, 



Any* instrument may be used in the bridge system without 

 alterations being made ; but in the differential system the 

 coils must be differentially wound, or some equivalent device 

 employed, so that two currents, one passing through each coil, 

 may annul each other's action on the magnet or cores within 

 them. On the other hand, only one balancing resistance is 

 required, instead of the three in the bridge system. The fol- 

 lowing diagram (fig. 2) is a theoretical view of the differential 



Fig. 2. 

 ix Station A. 7 < Station,!? &, 



*^-f 



I 



system. g, g at station A and g r , g r at B are the coils of the 

 receiving instruments, / and/' the batteries, and x, x f the 

 balancing resistances. We shall suppose, as is usually the 

 case, that/™/' and g—g'. To find the strength of the signals 

 each station receives from the other, let both send the same 

 current to line ; then, from the identity of the arrangement at 

 each end, there will be no current in the line, in, the right-hand 

 coil at station A, and in the left-hand coil at B. Therefore, if 

 ,JS is the strength of the signal, 



tmjMrt* ..... (io) 



where E is the electromotive force of the battery, m the 

 strength of signal produced by the unit current circulating- 

 through a single coil of the receiving instrument, and x the 

 external resistance. The value of x is 



" g + <y+ ffg + *0 (n) 



