ELECTRO-MAGNETIC ALARUM. 



Fig. 60. 



current, but by the force of a mainspring or descending weight, 

 transmitted to the hammer or tongue in the same manner exactly 

 as that in which the force of a mainspring or weight of a clock is 

 transmitted to the striking apparatus. The current does nothing 

 more than disengage a catch by which the motion of the wheel- 

 work acted on by the mainspring or weight, is arrested. The 

 catch once disengaged, the action of the current on the bell ceases, 

 and the ringing is continued by the action of the mainspring or 

 weight, and it may in like manner be stopped by the current 

 again throwing the catch between the teeth of one of the wheels. 



It will, therefore, be apparent that since the force which 

 impels the bell is independent of the current, a bell of any desired 

 magnitude may be acted upon by a hammer of any desired weight, 

 without requiring any more force from the current than that 

 which is sufficient to enable the electro-magnet to disengage the 

 catch by which the mechanism of the bell is arrested. 



158. Although the bell mechanism used for telegraphs differs in 

 nothing which is essential from that of a common alarum clock, it 

 may not be without interest to show one of the varieties of 

 mechanism in practical use. 



In fig. 60 is given a view of 

 the bell mechanism, as used on 

 the telegraphic line of the South- 

 Eastern Railway Company.* 



A is the electro-magnet. 



B its armature. 



B e a lever attached at the 

 upper end to the armature, and 

 having at the lower end a catch, 

 e, which when the armature is not 

 attracted towards the magnet is 

 pressed by a spring, /. 



d a wheel having a tooth in 

 which the catch e is engaged by 

 the pressure of the spring/, when 

 the armature B is not attracted 

 towards the magnet, but which is 

 liberated from the catch e, when 

 the armature B is drawn towards 

 the magnet. 



a a cylindrical box containing a strong mainspring, by which 

 the train of wheelwork is kept in motion so long as the catch e is 

 not engaged in the tooth of the wheel d. 



Elect. Tel. Manip., p. 23, 



205 



