THE ELECTRIC TELEGRAPH. 



the hand upon the dial at the station to which the message is sent 

 will also turn to the letter M, and in this way, by merely directing* 

 the hand successively to the letters of a word, pausing a little 

 while at each letter the word will be spelled to the agent at the 

 distant station. 



All alphabetic telegraphs, whatever be their form or construc- 

 tion, convey the communications in this manner. 



The French railway telegraph is in its principle identical with 

 the state telegraph. The indicator in the latter makes a complete 

 revolution by eight successive steps, moving in each step through 

 an angle of 45. If the alphabet consisted of only eight letters, 

 this would at once become an alphabetic telegraph by fixing the 

 indicator in the centre of a dial upon which, at equal distances 

 asunder, the eight letters are engraved. But since the French 

 alphabet consists of 25 letters, and since an additional sign is found 

 convenient, the dial is divided into 26 equal arcs instead of eight, 

 and the indicator makes a complete revolution by 26 equal motions, 

 at the termination of these motions respectively pointing to the 

 letters engraved upon the dial. 



To accomplish this, the escapement wheel is constructed with 

 13 teeth instead of 4, the groove upon the moveable disc of the 

 commutator has 13 sinuous undulations instead of 4 sides with 

 rounded corners, and the fixed disc upon which the handle of the 

 commutator moves, has 26 notches instead of 8. 



The grooved disc, by the motion of which the oscillations right 

 and left are imparted to the lever which makes and breaks the 

 connection with the battery, is fixed immediately behind the 

 notched disc, and the sinuous groove has the form represented in 

 fig. 51, and acts upon the lever in the manner described in 133. 



The commutator, with its appendages, is represented in fig. 73. 

 The fixed disc has at its edge 26 notches, into which the pin 

 projecting from the handle falls, as in the state telegraph. 

 Engraved upon the face of the disc are, on the outside, the numbers 

 from to 25, and on the inside the 25 letters (W being omitted, 

 not being generally used in the French language), the 26th place 

 having the mark -j- 



A part of the dial is broken away, to disclose the face of the 

 moveable disc, with the sinuous groove behind the fixed disc. 

 The lever G is visible, with its pin in the groove, and the oscilla- 

 tion of the end of the lower arm H between the contact-pieces,, 

 p and P', is exactly the same as that described in 133 and 

 in 184. 



The handle of the commutator is keyed upon an axis which, 

 passing through the centre of the fixed dial, is itself keyed into- 

 the centre of the moveable grooved dial behind it, so that when. 

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