DETECTOR OF FAULTS. 



divided at o, between the two wires o G and o G', but the chief 

 part will flow by the shortest and easiest route, and the galvano- 

 meter G' will be very much, and G very little, deflected. This will 

 show that F must be very much more than 10 miles from the 

 station. The arm A c will then be turned successively from bobbin 

 to bobbin. AVhen directed to the second bobbin, the current on 

 o G' will have the same intensity as if it flowed on 20 miles of 

 line wire, when turned to the third the same as if it flowed on 30 

 miles of line wire, and so on. The needle of G' will, therefore, 

 continue to be more deflected than that of G, although the dif- 

 ference will be less and less, as the number of bobbins brought 

 into the circuit is increased. When the bobbins included 

 represent 140 miles, G' will be a little more, and when they 

 represent 150 miles it will be a little less deflected than G, from 

 which it will be inferred that the point of fault lies between the 

 140th and the 150th mile from the station. A closer approxima- 

 tion may then be made by the introduction of shorter bobbins, and 

 this process may be continued until the place of the fault has been 

 discovered with all the accuracy necessary for practical purposes. 



69. It appears nevertheless, that in the practical working of the 

 telegraphic lines, occasions for the application of these expedients 

 are of extremely rare occurrence. During the four winter 

 months of November, December, January, and February 1853-54, 

 distances of 300 miles of underground wire, without any break of 

 circuit, have been in constant operation under the Magnetic 

 Telegraphic Company, and notwithstanding an unusual pre- 

 valence of unfavourable weather, with frequent and continued 

 snow-storms, no stoppage whatever has taken place. 



70. The Prussian underground lines of wire have been attended, 

 however, with occasional failures, which have produced some public 

 inconvenience. This circumstance has been ascribed to the faulty 

 method of laying the wires. The gutta-percha enveloping them 

 was mixed with sulphur, a process called Vulcanisation. Upon 

 being deposited in the ground the sulphur was soon abstracted, 

 leaving the gutta-percha brittle and porous. 



71. The under-ground line of the European and Submarine 

 Company, from London to Dover, is laid down in nearly the same 

 manner as that of the Magnetic Company. There are six conduct- 

 ing copper wires encased in gutta percha. To detect the more 

 easily the place of any accidental breach of continuity, a box 

 is placed at the end of each mile, in which a few yards of the 

 continuous line of wire are coiled, so that in case of any accidental 

 interruption occurring to the flow of the current, the particular 

 mile in which that interruption exists can always be ascertained 

 by putting the coils at the end of each successive mile in 



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