566 



THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



Let us examine the different cases that may present themselves, and 

 see how the post office clerk will act under these circumstances. The 

 apparatus being at rest, the tongues of the commutators are upon 

 s and s', (see Fig. 360), to which are joined the wires of the two alarums. 

 If the clerk at Paris wishes to send a message to Sevres he makes the 

 handle of the manipulator pass over an entire circle ; the current thus 

 sent into the line enters the station at Sevres by the right hand wire, 

 deflects the needle of the galvanometer, and acts on the mechanism of 

 the right alarum. Warned by the noise, the clerk puts the commutator 



FIG. ct>4. A dial telegraph station. 



to the right on m', and then, making the handle of his manipulator 

 describe a whole circle, a similar motion of the indicating needle is 

 caused at Paris, and in this way he announces that he is ready to 

 receive the message. The message sent and understood, the clerk at 

 Sevres sends in his turn the two letters c (compris). 



To send numbers, the letter c twice repeated is sent first. 



"What w r e have just said will enable the reader to understand the 

 plan which the clerk at Sevres must follow if he wishes to send a 

 message to Paris. The explanation is entirely the same, except the 



