CHAP, iv.] ELECTRIC TELEGRAPHY. 675 



CHAPTEE IV. 



ELECTRIC TELEGRAPHY (continued). 



I. WRITING TELEGRAPHS. THE MORSE AND MORSE-DIGNEY 



TELEGRAPH. 



THE needle and dial telegraphs just described form a pretty numerous 

 system, each of which has its own advantages and drawbacks. In 

 the first, which are very simple in construction, a feeble current is 

 sufficient, but they are very susceptible of disturbance. The second 

 cl^ss, the mechanism of which is far more complicated, have the ad- 

 vantage of being easily worked after only a short apprenticeship. 

 Both of them, however, are subject to a grave defect, they leave no 

 trace of the message sent to control its correctness, in case of a false 

 interpretation, interruption or fraud. 



The Morse telegraph which dates from 1838 is the type of the 

 writing telegraphs. The universality of its adoption on the great 

 majority of telegraphic lines is justified by the simplicity of its 

 mechanism and the certainty of its indications. We will first describe 

 the Morse apparatus itself, and will then indicate the modifications 

 it has received : to the notable improvement of the signals. 



The manipulator is represented in Figs. 371 and 372. It is 

 composed of a wooden base on which are fixed two binding screws, 

 & and d, and in the middle a short forked column, between the 

 branches of which a lever A can oscillate in a vertical plane. To the 

 screw d is attached the wire P which comes from the positive pole of 

 the battery ; b communicates with the wire R which ends in the 

 indicator and the column in the middle receives the line wire L. The 

 lever A is provided, at each of its extremities, with two screws a and 



