THE ELECTEIC TELEGEAPH. 



will presently appear, that the position in which the rod is stopped 

 determines the signal transmitted, it would follow as a consequence 

 that in such case all the keys would transmit the same signal, 

 and the indicator at the station to which the dispatch is to be 

 transmitted would always return to the same letter upon the dial. 



To prevent this, and to vary the signal in the necessary manner, 

 the projecting arms are inserted in the steel rod according to a 

 spiral or heliacal line, surrounding it like the thread of a screw, 

 so that if, for example, the rod be placed so that the first projecting 

 arm corresponding to the key marked with the cross, points directly 

 upwards, the fourteenth which corresponds to the key M, will 

 point directly downwards, and the intermediate arms will point at 

 angles more and more inclined from the upward direction, each 

 being deflected from the upward direction more than the preceding 

 one by the fourteenth part of the half circumference. 



In like manner, in proceeding from the arm corresponding with 

 the key M, which points downwards, each successive arm will be 

 more and more deflected from the downward direction, each being 

 more deflected from it than the preceding one by the fourteenth 

 part of half the circumference. 



Thus the twenty-eight projecting arms divide the circumference 

 of the rod into twenty-eight equal parts, and consequently in a 

 revolution of the rod, the arms come successively to the position 

 in which they point upwards and in which they would encounter 

 the pin projecting from the bottom of the key if that pin were 

 thrown in their way by the key being pressed down by the finger. 



It will be evident, therefore, that if from any cause the steel 

 rod be made to revolve, its motion may be stopped at twenty-eight 

 different points of its complete revolution by means of the depres- 

 sion of the twenty-eight keys. We shall now show how a motion 

 of revolution is imparted to this rod. 



To its right-hand extremity is fixed a ratchet-wheel, which 

 is in connection with a train of clockwork, moved in the usual 

 manner by a mainspring. This clockwork is contained within 

 the case of the apparatus. If it be wound up, and if nothing 

 obstructs its action, a motion of continuous rotation will be im- 

 parted to the ratchet-wheel, and by it to the steel rod, and this 

 motion will be more or less rapid according to the force of the 

 mainspring, and the adjustment of a fly which is connected with 

 it. They are so adjusted as to cause the rod to revolve two or 

 three times in a second. But in the teeth of the ratchet-wheel, a 

 catch is inserted, which counteracts the mainspring and preventf 

 the motion, which can only take place when this catch is with- 

 drawn. A bar is suspended parallel to the keys, and under them, 

 by a contrivance called in mechanics a parallel motion, by meana 

 40 



