THE ELECTRIC TELEGRAPH. 



Meritorious inventions sometimes neglected. 179. Needle instruments 

 generally used in England. 180. Single needle instrument. 181. 

 Double needle instrument. 182. Old aerial telegraph. 183. French 

 State telegraph. 



163. THE momentary currents in the one direction or in the 

 other will, therefore, be produced upon the wire connected with 

 the extremities of the coil, such as have already been described, 

 each time the poles, :N T and s, are presented to and withdrawn 

 from the ends, a and 6, of the horse-shoe of soft iron. If the 

 magnet, N o s, were mounted so as to revolve upon an axis passing 

 through the centre of its bend, and therefore midway between its 

 legs, its poles might be made to pass the ends of the horse-shoe, 

 the latter being stationary. During each revolution of the 

 magnet, K o s, the polarity imparted to the horse-shoe would be 

 reversed. 



When the pole N approaches b, and consequently s approaches a, 

 south polarity will be imparted to &, and north polarity to a ; 

 and when N passes a, and consequently s passes &, south polarity 

 will be imparted to a, and north polarity to b. 



The momentary currents produced by these changes of mag- 

 netism in a and b will be easily understood by what has been 

 explained. When N approaches &, and s approaches a, the com- 

 mencement of south polarity in 5, and north polarity in a, will 

 both impart to the wire a current in the same direction, because 

 the coils of the spiral as presented to s will be the reverse of 

 those presented to N. When N departs from 6, and s from a, the 

 cessation of south polarity in 6, and of north polarity in , will 

 impart currents in the same direction to the wire, but this 

 direction will be opposite to that of the former currents. 



When N approaches a, and consequently s approaches 5, cur- 

 rents will be imparted to the wire whose direction will be the 

 same as that of those produced by the departure of js from b, and 

 of s from a. When N departs from a, and s from b, currents will 

 be produced in the same direction as when K approaches b and s 

 approaches a. 



If the direction of the currents produced when N approaches 

 b, and s approaches a, be indicated by an arrow directed to the 

 right, and that of those produced when N departs from b, and s 

 from a, by an arrow directed to the left, the changes of direc- 

 tion which take place in each revolution of the magnet N o c, 

 will be such as are indicated in fig. 63, where b and a 

 represent the ends of the horse-shoe, b a ; IT the position of the 

 pole in approaching, and N' in departing from b, and u ff its 

 position in approaching, and y ff in departing from a. The 

 arrows directed to the right express the direction of the two 

 2 



