CHAP. HI.] ELECTRIC TELEGRAPHY. 547 



sufficient for this purpose to arm the latter with a spring which keeps 

 it at a short distance from the poles, without preventing it coming in 

 contact with them every time the current passes. On this principle is 

 based the construction of the machines known as electro-magnetic 

 engines, because electricity is the source of the motion they produce. 

 This motion, which it has been attempted to utilize for purely mecha- 

 nical purposes, as we shall see in a future chapter, serves for the 

 production of signals which can be transmitted with very great 

 rapidity to considerable distances, owing to the enormous velocity 

 with which electricity is propagated in a conducting wire. Such, 

 reduced to its simplest form, is the method of producing motion most 

 generally adopted in the different systems of electric telegraphy. 



Nevertheless, in certain of these systems, the electric current acts 

 either directly on the needles of a galvanometer or indirectly by its 

 chemical or electrolytic properties. But whatever in other respects 



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FIG. 348. Electromagnets. 



may be the mode of action of the electricity, an electric telegraph 

 is always necessarily composed of the four following parts : 



First, an apparatus for producing the current, that is to say, an 

 electro-motor. This is sometimes a galvanic battery, sometimes an 

 induction machine, either magneto-electric or electro-magnetic. 



Secondly, an apparatus of transmission, forming a circuit , or electro- 

 dynamic conductor. This is the wire or wires of the line joining the 

 sending and receiving stations of the signals. 



In the third place an apparatus for producing the signals, called a 

 manipulator, which is handled by the person sending off the message. 



In the last place, there is a receiving apparatus, by which the 

 signals sent are reproduced at the receiving station ; this is called the 

 indicator or receiving instrument. 



We shall see presently that in an electric telegraph there are other 



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