JEDISON-'S ACOUSTIC INVENTIONS, 



131 



upon the pressure of the carbon, which is constantly changing when 

 the diaphragm is in vibration. 



Another form, acting on much the same principle, is illustrated by- 

 Fig. 6. It is called the inertia-telephone, though it is hardly certain 

 that its action is to be attributed solely to inertia. The carbon, C, is 

 placed between two metallic plates, one of which is fastened to the 

 diaphragm, and the other is held by a screw, bearing in a framework 

 attached to the diaphragm by insulating supports. 



Fig. (j. 



Fig. & 



Fig. 7. 



When vibrating, the whole system moves, instead of the plate P 

 alone, as in the ordinary carbon-transmitter. Mr, Edison's explanation 

 of its mode of action is, that the degree of pressure with which the 

 carbon rests against the plates is varied during the vibration. Thus, 

 after a movement toward the right, the diaphragm suddenly stops and 

 the carbon presses, in virtue of its inertia, on the plate P. 



An advantage which the magneto-telephone has over the carbon- 

 telephone is that its diaphragm does not touch anything, and can 

 therefore vibrate with perfect freedom. On the other hand, the dia- 

 phragm of the carbon-telephone presses with considerable force upon 

 the carbon. In the form shown in Fig. 7 this difficulty is not encoun- 

 tered. 



The diaphragm carries an armature. A, of soft iron, which con- 

 fronts but does not touch the magnet P. A and P are opposite poles 

 of the same magnet, being connected at P and polarized by a local 

 circuit. The magnet P presses upon the carbon at C, the pressure 



