Discharge of long Overland Telegraph Lines. 27 



resistance must be equal to the resistance of the coils of the dis- 

 charging relay. This law will hold good for any long overland 

 telegraph line ; and it is only for a long line that such a dischar- 

 ging arrangement is required. 



As regards the absolute value of r, it was found that 200 

 S. units, using a Siemens'^ polarized relay, were quite large 

 enough. Such a relay works safely with 30 Minotti's cells 

 through 10,000 S. units. 



The shunt itself, even without having iron in it, produces an 

 extra current which is in the same direction in the shunt as the 

 primary current, and consequently opposes the extra current pro- 

 duced by the coil r in the closed circuit (r + w). 



In order to have, therefore, the action of the coil r not too 

 much lessened by the extra current produced by the shunt, it is 

 necessary to make the latter of the thinnest possible German- 

 silver wire, and wind it on a large bobbin with the convolutions 

 as far distant from one another as possible. Another method 

 would be to wind the bobbin bifilarly. 



In conclusion I may mention that the longest main line in 

 India is the one between Calcutta and Kurrachee, 1700 miles in 

 length, which has been worked direct now r for more than two 

 years, — Agra (which is about at the middle) only in translation. 

 During the dry season, when the lines up country often have an 

 insulation of more than 200 millions S. units per mile, it is pos- 

 sible to work this enormous distance altogether direct without 

 Agra in translation ; but practically nothing would be gained by 

 this, since then, on account of the great length, the charge be- 

 comes so large as to reduce the speed to less than fifteen words 

 a minute, while by having Agra in translation the speed, if only 

 the signalling system would allow of it, would reach to upwards 

 of sixty words a minute. 



Note. — Mr. A. Cappel, in his report on the Central London Office 

 of the Electric and International telegraph Company, states that a 

 shunt in connexion with an electromagnet for discharging one of the 

 cables was made use of as early as 1867, and was announced to him 

 by Mr. Culley as an invention of one of the telegraph clerks. This 

 appears to be the first application of the extra current for this pur- 

 pose ; but I am not aware whether this simple principle has since been 

 used for overland telegraph lines. 



Mr. Cappel says : — "The duration of the zinc current (necessary 

 to neutralize, after each signal sent, the positive discharge of the 

 cable) can be regulated by varying the resistance of the shunt ; but no 

 definite law or conclusion has yet been arrived at on the subject." 



