622 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK 



IV. THE ALARUMS. 



The alarums, whose office we have indicated in describing the 

 telegraphic apparatus, may now be referred to without entering into 

 any detail as to the mechanism which sets them in action. The 

 systems of alarums are at least as numerous as those of telegraphic 

 apparatus themselves. We must limit ourselves to the explanation 

 of one or two of these systems. 



The simplest and most generally adopted on French telegraphic 

 lines is that of which Fig. 407 gives an interior view. An electro- 

 magnet receives in its bobbin the current reaching the screw A, and 

 thence by the handle of the hammer BM, in contact with a spring R, it 

 goes by the button I, and the screw D, which communicates with the 

 batteries, and the circuit is then closed. The hammer rod which acts 

 as armature is attracted by the electro-magnet and the hammer 

 strikes the bell. But the contact with the spring is by this time 

 stopped : the current is interrupted, the hammer rod falls again upon the 

 spring, which gives rise to a fresh current and so on, as long as the 

 current passes through the alarum, that is to say, as we have seen in 

 describing Breguet's dial telegraph, as long as the commutator is on 

 the corresponding button. 



The result is a series of repeated blows, very close to one another, 

 whence the name of vibrating alarum, given to the apparatus. The 

 principle of the mechanism is due to Neef, and it was a Belgian 

 electrician, M. Lippens, who first applied it to alarums. 



When more prolonged and intense alarums are required, a catch 

 mechanism is adapted to the system of vibrating alarum described 

 above, which introduces into the apparatus the circuit of a local 

 battery. Such is the alarum of M. Aubine (Fig. 408) in which a bent 

 lever is held against the handle by a lateral tooth. When the alarum 

 is set in action by the current of the line, the hammer being attracted 

 by the electro-magnet, disengages the lever, which then falls on to the 

 spring, ?', and leaves r. It is easy to see then that the current from 

 the line is broken, while that of the local battery PN is closed. The 

 alarum is thus set in action by a more powerful current, which con- 

 tinues as long as the clerk warned does not return the catch-lever to its 



