THE ELECTRIC TELEGRAPH. 



Fig. 55. 



passing through the frame supporting the coil, and through the 

 dial plate, supports in front of the dial the hand a' &', which is 

 fixed upon the axis in a position parallel to 

 the needle a b, so that it must play before the 

 dial in a manner corresponding exactly with 

 the play of the needle a b within the coil. 



141. In order to govern the play of the 

 needle, it is necessary that the agent at the 

 station from which the signal is transmitted 

 should have the power, 1st. To suspend and 

 transmit the current at the receiving station ; 

 and 2nd. To change its direction upon the 

 conducting wire. The former is necessary, to 

 enable him to bring at all times the needle to 

 its position of rest ; and the latter, to deflect 

 it to the right or to the left, according to the 

 exigencies of the telegraphic communications. 

 The general principle on which these changes 

 in the flow and direction of the current are 

 effected, has been already explained (111). It is easy to imagine, 

 that by very simple mechanism the movement of a lever or arm 

 may make or break the contact of the conducting wires, so as to 

 transmit or suspend the current at pleasure. Also by a simple 

 motion of such an arm the hands, A and A", fig. 48, or any equivalent 

 pieces, may be moved from p to N and from N to P, so as to reverse 

 the current upon the wire to which the arm A' is directed. 



If then an agent at the station, s", for example, be provided with 

 any means of suspending or reversing the current which passes 

 along the wire, between s and s", he can at will bring a magnetic 

 needle, mounted at s, to its position of rest, that is, to the vertical 

 position, by suspending the current or deflect it to the right, by 

 causing the current to flow in one direction on the conducting 

 wire, or to the left, by reversing the direction of the current. 

 The particular manner in which these several operations sub- 

 serve to telegraphic purposes will be pre- 

 sently explained. 



142. To explain the manner in which the 

 electric current can impart temporary mag- 

 netism to soft iron, let us suppose a copper 

 wire wrapped with silk, to prevent the me- 

 tallic contact of contiguous convolutions, to 

 be coiled round a rod of soft iron, bent into 

 the form of a horse-shoe, as represented in 

 fig. 56, care being taken, that in carrying 

 the wire from one arm to the other, the 



Fig. 56. 



