﻿16 L. Schwendler — O71 the general Theory of Duplex Telegrajphy. [No. 1, 



f 



— . If it is, however, considered that the time during which this error lasts 

 2i 



is very small compared with the time it takes to make a signal, its disturbing 

 effect will never be appreciable in practice, i. e. p will remain sensibly con- 

 stant during the time the kej is moved to produce a signal. 



There will be no practical difficulties connected with the fulfilment of 

 equation (VII), and therefore also none with the fulfilment of equation 

 (VI) ; for y8 the internal resistance of the signalling battery is the only 

 quantity which of itself can alter in time. However, this variation of ^ for 

 any efficient form of signalling battery being invariably steady and small, 

 it will be always possible to neutralize its action in time by a simple re-ad- 

 justment of w. 



If Leclanche's cells are used, or well prepared Minotti's, a weekly ad- 

 justment of w should be sufficient. The measuring of /5 will always be an 

 easy matter.* 



Hiyid fulfilment of the 2nd Condition, i, e., S = 0. 

 The general expression for S' was 



W m" , ^ E'b'' , , 



^=~¥v-^^ -^+"^ (1^') 



Remembering that by equation (VII) 



w/ + /5/ = f/ 

 we know that \pl =^ cf>', and substituting further for a its value, the general 

 expression for S' becomes : — 



E"m" , E'b' , f E' m' E" m" ) , 



S = -N^/^^- — + j-^T---^-/]^' (IV) 



and this form of S shews at once that it is perfectly immaterial for duplex 

 working by the Bridge method whether the same or opposite poles of the 

 two signalling batteries be put to line,t for in both cases equation (IV) 

 becomes : 



E' m' .. „.b 



^' = -N7-^'-^'i7 (IV) 



Further, it will be seen that the right hand member of equation (IV) 

 * My friend Mr. R. S. Brough suggested the foUowing very simple method for keeping 



w+ )8 = f (VII) 



Insert a small galvanoscope in the branch b, for which balance is established with 

 respect to the received current, i. e. 



ad— fg = (VI) 



Now note the deflection on the galvanoscope when both stations are sending simul- 

 taneously, and again when the station for which ^ is to be measured is sending alone. 

 Then clearly if these two deflections are equal, w + j8 must be equal to/. If the two 

 deflections are not equal then alter w until they become equal. After the determination 

 is made, the galvanoscope is short circuited. 



t In practice however I prefer to put the same, namely, the positive poles to the 

 line, as then defective insulation will not be felt so much. 



