THE ELECTRIC TELEGRAPH. 



the key is placed, it is necessary to leave a free passage for the 

 current along the line-wires m n. To effect this, the screw <7, 

 which passes through the short arm of the key, is turned so as to 

 raise the short arm, and consequently lower the arm E D until the 

 hammer v is brought into permanent contact with the anvil q. 

 When that takes place, the metallic continuity between m and n 

 will be established, and the current will flow without interruption 

 on the line-wire. Whenever it is desired to transmit a dispatch, 

 the screw d is turned so as to lower the arm d, and to raise E D, 

 and thus to raise the hammer from its contact with the anvil. 

 The key is then ready for the transmission of the dispatch in the 

 manner already described. 



129. In some telegraphic apparatus it is necessary to make the 

 intervals of transmission and suspension of the current absolutely 

 equal in duration, and to succeed each other with chronometric 

 regularity. There are many expedients by which this can be 

 accomplished, of which the following is an example. 



A metallic wheel put in connection with clock-work, so as to 



Fig. 49. 



receive a regular motion of rotation, has its edge divided into 

 equal parts by pieces of ivory, or some other non-conductor inlaid 

 upon it, as represented in fig. 49, where m represents the metal, 

 and i the ivory. A metallic spring r' connected with one end of the 

 conducting wire w', presses constantly upon its edge ; and another 

 r connected with the other end of the wire w, presses constantly 

 on the metallic axle of the wheel which is otherwise insulated. 



Now, if the wheel be supposed to have an uniform motion of 

 revolution, the alternate divisions of ivory and metal on its edge 

 will pass in succession under the spring r', while the spring r will 

 be in constant metallic contact with the axis. If a current flows on 

 the wire w, it will be transmitted by the spring r to the axle, and 

 188 



