THE ELECTEIC TELEGRAPH. 



The actual contact of the armature B with the poles of the 

 electro-magnet is prevented by two small ivory knobs screwed into 

 the surface which is presented to the magnet. The play of the 

 armature B is so limited that the catch e shall be just disengaged 

 from the tooth of the wheel d, when the ivory knobs come into 

 contact with the poles of the magnet. 



When the wheel- work is liberated by the magnet withdrawing 

 the catch e from the wheel d, the mainspring in the cylindrical 

 box a causes the toothed wheel attached to the box to revolve. 

 This wheel drives a pinion on the axle of the wheel 6 ; the wheel 

 b drives a pinion on the axle of the wheel c ; the teeth of the 

 wheel c are engaged with those of a pinion on the wheel d. 

 The movement of the train is stopped, when the catch e falls 

 under the tooth of the wheel d. The wheel i, which is engaged 

 in the anchor of the escapement g, is fixed upon the axle of 

 the wheel c, turns with the latter, and thus gives an oscil- 

 lating motion to the anchor, which is imparted to the hammer h 

 of the bell D. The bell is therefore acted upon by the hammer so 

 long as the magnet A keeps the catch e from falling under the tooth 

 of the wheel d. 



159. Since the magnitude, loudness or pitch of the bell is 

 independent of the force of the current, the telegraphic offices are 

 provided with various bells for special purposes. 



Sometimes a special wire is appropriated to the bell which is 

 acted upon by a special current. 



In other cases the regular current intended to work the tele- 

 graph is diverted to the bell apparatus by the commutator. In 

 other cases again, the object is accomplished by the expedient 

 explained in (134), which is known as the short circuit. 



160. Having explained the form and construction of electro- 

 magnets, we are prepared to show the manner in which an electric 

 current may be produced by the mere action of magnets without 

 any intervention of a voltaic battery. 



The electricity thus produced has been called MAGNETO- 



ELECTEICITY. 



161. Let a silk or cotton covered wire be coiled heliacally on a 

 roller or bobbin having a hollow core of sufficient magnitude to 

 allow a cylindrical bar to be passed into it. Let the covered wire 

 be coiled constantly in the same direction, beginning from A B, 

 (fig. 61), and terminating at c D. Let the extremities mn of this 

 wire be joined to those of a wire m o n of any required length, 

 stretched to any required distance. Now let the north pole 

 N of a magnet s N be suddenly passed into the core of the 



206 



