CHAP, v.] TELEGRAPHIC LINES. 617 



the condensed electricity can pass to the earth on both sides of the 

 line. In this way the charge that is of opposite kind to that which 

 furnished the first current of electricity working on the indicator, 

 combines with the latter across the cable and neutralizes it, instantly 

 destroying at the same time the inductive effect produced by it in the 

 covering of the cable. In this manner the cable is restored, almost 

 instantaneously to the neutral state, and becomes susceptible 

 immediately of a new signal." 



The system, however, which is adopted on the great transatlantic 

 lines is this. The telegraphic apparatus is a needle one, the reason of 

 this choice being the extreme sensibility of the galvanometers, whose 

 needles will oscillate under the action of very feeble currents. Never- 

 theless, to increase still further this sensi- 

 bility, and to enable the clerks of the re- 

 ceiving station to read the signals without 

 hesitation, Thomson's galvanometer is em- 

 ployed in the following way : " In this 

 apparatus," says M. du Montcel, " the 

 sensitive part is a little lenticular mirror 

 directed magnetically by a little magnetized 



needle, which is itself drawn back into a g jpr^Jl^a * 

 fixed position by a magnet ; a ray of light is 

 thrown on this little mirror, and reflected FIG. 4os.-section of Thomson's 



galvanometer in the telegraphic 



by it on a screen placed at a distance of Sffe1t I Brest he trausatlantic 

 eight 'feet. With this amplification, the 



least motion, imperceptible to the naked eye, is manifested by the dis- 

 placement of the projected image, and the positions that this image 

 successively occupy, to the right or left of a fixed datum line, 

 indicate the dots and dashes of Morse's alphabet. All the com- 

 binations necessary for the interpretation of the messages are thus 

 obtained, being read upon the screw in a darkened chamber." 



Figures 403 and 404 represent the telegraphic apparatus of the 

 French transatlantic cable, as it is set up at the station at Brest. 

 The first is a section of Thomson's galvanometer, the second shows 

 the general arrangement of the apparatus. In the centre of the 

 bobbin we see a little circular mirror, which carries the magnetized 

 needle rendered astatic by the magnet E, fixed to a vertical rod above 

 the galvanometer. A silk thread supports the mirror, whose motions 



