THE ELECTEIC TELEGRAPH. 



will transmit the current with a certain intensity depending on 

 its length, its thickness, and, in fine, on the conducting power of 

 the metal of which it is made. 



Now let us suppose that a certain length of the wire of the 

 telegraphic line be taken, which will transmit a current of the 

 same intensity. A galvanometer placed in each current will 

 then be equally deflected. But if the length of the line wire be 

 less or greater than the exact equivalent length, the galvanometer 

 will be more or less deflected by it than it is by the bobbin wire, 

 according as its length is less or greater. 



It is, therefore, always possible by trial to ascertain the length 

 of line wire, which will give the current the same intensity as 

 that which it has upon any proposed bobbin wire. 



Bobbins may therefore be evidently made carrying greater 

 or less lengths of wire upon which the current will have the 

 same intensity as it has upon various lengths of line wire. 



Suppose then a series of bobbins provided, which in this sense 

 represent various lengths of line wire from 100 feet to 300 miles, 

 and let means be provided of placing them in metallic connection 

 in convenient cases. 



Such an apparatus is that by which the Messrs. Bright detect 

 the points of fault. 



Let B be the station battery, G a galvanometer upon the line 

 wire, F the point of fault at which the current escapes to the 



Fig. 24. F 



earth, in consequence of an accidental defect of 

 the insulation. Let a wire be attached to the line 

 wire of the station, at o, and let it be connected 

 with the first of a series of bobbins such as are 

 described above ; let a galvanometer, similar to G, 

 be placed upon it at G'. Let a metallic arm A c, 

 turning on the point A, be so placed that its ex- 

 tremity c shall move over the series of bobbins, 

 and that by moving it upon the centre A, the end 

 c may be placed in connection with the wire of 

 any bobbin of the series. Let A be connected by 

 a conducting wire with the earth at E', the nega- 

 tive pole of the battery B being connected with the earth at E. 



The apparatus being thus arranged, let us suppose that the 



wire A c is placed in connection with the first bobbin, representing 



10 miles of the line wire, and that the distance G F of the point of 



fault is 145 miles. In that case the battery current will be 



143 



