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.
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 diaphragm of the carbon-telephone presses with considerable force upon the carbon. In the form shown in Fig. 7 this difficulty is not encountered.
The diaphragm carries an armature. A, of soft iron, which confronts but does not touch the magnet B. A and B are opposite poles of the same magnet, being connected at P and polarized by a local circuit. The magnet B presses upon the carbon at C, the pressure
