574 THK APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



automatically with the number of letters or signals between the signal 

 button releasing the arm and the signal button at which the arm is 

 once more arrested. If therefore the index-pointer of the communi- 

 cator is at zero on the dial, and the next finger button opposite A is 

 depressed, one current passes into the line, and A will be shown on 

 the distant dial : the button opposite the letter N being now depressed 

 the A button rises up and liberates the revolving contact arm, and 13 

 alternating currents pass into the line, the arm cutting off the currents 

 again by contact with the lever of the N button : the D button being 

 now depressed the arm is again at liberty to rotate, and 17 alternating 

 currents are passed into the line before its motion is arrested at D ; 

 thus the pointer on the dial of the indicator of the distant instrument 

 which also stood at zero, will have advanced one space, showing A, 

 then step by 'step 13 spaces, stopping at N, and again 17 spaces to D, 

 indicating the word " AND." The " indicator" consists of a delicately 

 poised magnetic bar armature which is caused to oscillate between the 

 poles of four magneto-electric coils by the alternating currents passed 

 through the coils from the communicator ; the axis of this armature 

 carries a lever and very delicate escapement wheel, to the axis of 

 which is attached a pointer which rotates over an external dial 

 corresponding with the dial of the communicator, as this escapement 

 wheel oscillates between two fixed stops, it is impelled forward in one 

 direction step by step, and two hair-spring detents at each movement 

 lock the wheel, securing a dead beat motion to the index hand over 

 the dial, thus the alternating currents passed from the communicator 

 are reciprocated in the oscillations of the armature and escapement 

 arrangement of the " indicator," the index pointer of which is arrested 

 at the particular letter which the depressed button of the communicator 

 at the distant station represents. 



The " alarm " in its mechanism is a modification of the oscillating 

 magnetic armature and electro-magnetic coil arrangement of the 

 indicator, -releasing a detent, and clockwork causing a hammer to 

 strike a bell. 



