564 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



apparatus. At the base of the apparatus, resting on the stand, is 

 seen the electro-magnet, through the bobbins of which the current 

 sent through the line by the receiving station circulates. In front of 

 its poles is an armature of soft iron M, Fig. 363, carried by two screws, 

 between which it can oscillate about its horizontal bearings at the top. 

 When the current passes, it is attracted by the poles, then excited, of 

 the electro-magnet, and rests against them. When the current is in- 

 terrupted it leaves the poles by an opposite motion towards the front 

 of the indicator where the dial is fixed. This reciprocating motion of 

 the armature M is communicated by a special mechanism to the 

 indicating needle. It carries for this purpose a vertical rod L, which 



FIG. 363. Details of the mechanism in Brguet's indicator. 



oscillates like the armature, but in an opposite direction. This rod, 

 limited in its motions by two screws, carries at its end a pin c 

 which works in a fork /, so that the latter oscillates sometimes 

 forward, sometimes backward, and communicates its own oscillations 

 to a shaft ab, and so to the pallet pp' t whose special purpose is to 

 let go or stop the teeth of the escapement-wheel R, 



Suppose the indicator at rest, the indicating needle being on the 

 cross, the pallet p' is stopped against the tooth 1 of the wheel, and the 

 wheel is immovable. When an emission of the current takes place- 

 that is to say, when the needle of the manipulator advances from the 



