ELECTRO-TELEGRAPHY 



237 



a hook which faces the wheel r,, and by catching in a notch on its circumference, 

 keeps the train at rest. But when a current circulates through the coils e, the 

 armature is attracted, the hook is raised, the train is liberated, and the pendulum- 

 hammer vibrates anil strikes a succession of blows. B is a support carrying a small 

 spring, which reacts on the lever, and restores it to its normal position when the 

 magnetism ceases. This alarum is used for calling the attention of telegraph clerks. 

 It requires a little attention to keep up the proper adjustment between the spring on 

 the one hand, and the magnetic attraction on the other. 



The telegraph originally adopted and still largely used by the French Adminis- 

 tration, is somewhat akin to the alarum just described. It has a train of wheels, a 

 scape-wheel with four teeth, and a pair of pallets. There is, however, no pendu- 

 lum ; but the pallets are connected with the armature of an electro-magnet, in such a 

 manner that, for each attraction or repulsion of the armature, the scape-wheel is 

 liberated half a tooth ; for an attraction and a repulsion a whole tooth ; so that four 

 successive currents, producing of course four consecutive attractions and repulsions, 

 produce a whole revolution of the scape-wheel. The axis of the latter projects 

 through the dial of the instrument (fig. 810), and carries an arm a or b (fig. 811), 



810 



which, following the motion of the wheel, is able to assume eight distinct positions. 

 The apparatus is generally double, as shown in the figure ; and the signals are made 

 up of the various combinations of the eight positions of each of the two arms. The 

 arm is half black, the other half white. The position of the black portion is read off; 

 the white portion is merely a counter- 

 poise. "When only one half of the 

 dial, or one index is in use, the com- 

 binations are shown by producing 

 with the one index successively the 

 positions of the two, whose combina- 

 tion makes the signal, always giving 

 first the position of the left-hand 

 index, then that of the right. The 

 handles shown in front are the con- 

 tact-makers ; and are so constructed 

 that the position of the arm on the dial coincides with the position given to the 

 handle. Fig. 811 is a front view of the two arms ; part of the dial is supposed to be 

 removed, so as to expose the four-toothed-wheel already mentioned, and the pallets x 

 and y ; which, in their movement to and fro, allow of the semi-tooth advances of the 

 wheel. 



In these various applications of the electro-magnet, the armature has been of soft 

 iron, and the only action of the electric-magnet has been to attract it. It has been 

 withdrawn from the magnet after the electricity has ceased to circulate, either by its 

 own gravity, by a counterpoise, or by a reacting spring. We now come to a telegraph 

 that is well known and much used, Henley's magneto-electric telegraph, in which 



