☞ The ordinary telephone consists essentially of a device by which currents of electricity, produced by sounds through the agency of certain mechanical devices and exactly corresponding in duration and intensity to the vibrations of the air which attend them, are transmitted to a distant station, and there, acting on suitable mechanism, reproduce similar sounds by repeating the vibrations. The necessary variations in the electrical currents are usually produced by means of a microphone attached to a thin diaphragm upon which the voice acts, and are intensified by means of an induction coil. In the magnetic telephone, or magneto-telephone, the diaphragm is of soft iron placed close to the pole of a magnet upon which is wound a coil of fine wire, and its vibrations produce corresponding vibrable currents in the wire by induction. The mechanical, or string, telephone is a device in which the voice or sound causes vibrations in a thin diaphragm, which are directly transmitted along a wire or string connecting it to a similar diaphragm at the remote station, thus reproducing the sound. It does not employ electricity.