THE ELECTRIC TELEGRAPH. 



The agent, therefore, at the station, receiving the dispatch, 

 being warned by the bell that the agent at the station s is going 

 to send a dispatch, turns the piece a b into the horizontal position, 

 and the bell ceases to ring, the telegraph x receiving the dispatch. 



135. The manner in which the pulsations oi- the current are 

 produced, controlled, and regulated, by the operator at the station 

 s" being understood by these examples and illustrations, it will 

 next be necessary to show how they are made to produce signals 

 at the station to which the dispatch is transmitted, by which the 

 operator or observer there can be enabled to understand and 

 interpret the communication. 



The effects of the current which have been found most conve- 

 nient for this purpose are 



1st. Its power to deflect a magnetic needle from its position of 

 rest, and to throw it into another direction. 



2nd. Its power to impart temporary magnetism to soft iron, 

 this magnetism suddenly deserting the iron when the current is 

 suspended. 



3rd. Its power to produce the chemical decomposition of certain 

 substances. 



136. All forms of electric telegraph depending on one or other 

 of these properties of the current, it is indispensably necessary to 

 understand them before the reader can hope to comprehend the 

 mode of operation of these wonderful instruments. 



If a wire be extended over and under a compass-needle which 

 directs itself to the magnetic north and south, parallel to the 

 needle, and as close to it as it can be placed without actually 

 touching it, as represented in fig. 53, the needle will remain 

 undisturbed in its position. Let the ends p and n of the wire be 

 then attached to the poles of a vol- 

 taic battery, so that a current of a 

 certain intensity shall be transmitted 

 upon it. The moment the current is 

 established upon the wire, the mag- 

 netic needle a b will be thrown out 

 of its usual direction, and instead of 

 pointing north and south, it will 

 point east and west. 

 If the direction of the current upon the wire be reversed, the 

 direction of the deflexion of the needle will be reversed. 



Fig. 53. 



192 



