UNDERGROUND WIRES. 



Railway station to the Telegraph Company's offices in Exchange 

 Street, East, was laid in eleven hours ; and in Manchester the 

 line of streets from the Salford Railway station to Ducie Street, 

 Exchange, was laid in twenty-two hours. This was the entire 

 time occupied in opening the trenches, laying down the telegraph 

 wires, refilling the trenches and relaying the pavement. 



68. One of the objections against the underground system of 

 conducting wires, was, that while they offered no certain guarantee 

 against the accidental occurrence of faulty points where their 

 insulation might be rendered imperfect, and where, therefore, the 

 current would escape to the earth, they rendered the detection of 

 such faulty points extremely difficult. To ascertain their position 

 required a tedious process of trial to be made from one testing 

 post to another, over an indefinite extent of the line. 



A remedy for this serious inconvenience, and a ready and 

 certain method of ascertaining the exact place of such points of 

 fault without leaving the chief, or other station at which the 

 agent may happen to be, has been invented and patented by the 

 Messrs. Bright of the Magnetic Telegraph Company. 



Instruments called Galvanometers, which will be more fully 

 described hereafter, are constructed, by which the relative intensity 

 of electric currents is measured by their effect in deflecting a 

 magnetic needle from its position of rest. The currents which 

 most deflect the needle have the greatest intensity, and currents 

 which equally deflect it have equal intensities. 



The intensity of a current diminishes as the length of the con- 

 ducting wire measured from the pole of the battery to the point 

 where it enters the earth is augmented. Thus, if this length be 

 increased from twenty miles to forty miles, the intensity of the 

 current will be decreased one half. 



The intensity of the current is also decreased by decreasing the 

 thickness of the conducting wire. Thus the intensity, when 

 transmitted on a very thin wire, will be much less than when 

 transmitted on a thick wire of equal length ; but the thick wire 

 may be so much longer than the thin that its length will com- 

 pensate for its thickness, and the intensity of the current trans- 

 mitted upon it may be equal to that transmitted on the shorter 

 and thinner wire. 



The method of Messrs. Bright is founded upon this property 

 of currents. A fine wire wrapped with silk or cotton so as to 

 insulate it and prevent the lateral escape of the current, is rolled 

 upon a bobbin like a spool of cotton used for needle- work. A con- 

 siderable length of fine wire is thus comprised in a very small bulk. 



The wire on such a bobbin being connected by one end with the 

 wire conducting a current, and by the other end with the earth, 



L2 147 



