THE TELEPHONE. 



189 



diaphragm, and it begins to vibrate. The sound is not conveyed by the wire ; 

 the motion is communicated, and the vibrations become sound waves again. 

 The Telephone consists of a cylindrical magnet encircled at one end by a 

 bobbin, on which is wound a quantity of fine insulated copper wire. The 

 magnet and coil are contained in a wooden case, the ends of the coil being 

 soldered to thick copper wire, which traverse the " wooden envelope," and 

 terminate in the binding screws. In front of the magnet is a thin circular iron 

 plate, in which is the mouthpiece. The drawings will explain the instrument. 



Fig. 192. External appearance of Bell 

 Telephone. 



NO 



Fig. 193 a. Bobbin of coil wire round magnet, b. Dia- 

 phragm, c. Mouthpiece, d. Permanent magnet. 

 e. Wires to binding screws, f. Both wires as one for 

 convenience, g. Adjusting screw-holding magnet. 



Mr. Edison also invented a Telephone like Gray's, and made the 

 discovery, that when properly prepared, carbon would change its resistance 

 with pressure, and that the ratio of these changes corresponded with the' 

 pressure. This solved the problem of the production of speech. The 

 carbon is placed between two plates of platinum connected in the circuit 

 and near the diaphragm, and the carbon receives the pressure from it by 

 means of the mouthpiece. 



When we come to MAGNETISM and ELECTRICITY we may have some- 



