Conversion from Sound Waves to Vibration of Diaphragm.  However produced, by the voice or otherwise, sounds to be transmitted by telephone consist of vibrations of the air.  These vibrations, upon reaching a diaphragm, cause it to move.  The greatest amplitude of motion of a diaphragm is, or is wished to be, at its center, and its edge ordinarily is fixed.  The diaphragm thus serves as a translating device, changing the energy carried by the molecules of the air into localized oscillations of the matter of the diaphragm.  The waves of sound in the air advance; the vibrations of the molecules are localized.  The agency of the air as a medium for sound transmission should be understood to be one in which its general volume has no need to move from place to place.  What occurs is that the vibrations of the sound-producer cause alternate condensations and rarefactions of the air.  Each molecule of the air concerned merely oscillates through a small amplitude, producing, by joint action, shells of waves, each traveling outward from the sound-producing center like rapidly growing coverings of a ball.

Conversion from Vibration to Voice Currents.  Fig. 1 illustrates a simple machine adapted to translate motion of a diaphragm into an alternating electrical current.  The device is merely one form of magneto telephone chosen to illustrate the point of immediate conversion. 1 is a diaphragm adapted to vibrate in response to the sounds reaching it. 2 is a permanent magnet and 3 is its armature.  The armature is in contact with one pole of the permanent magnet and nearly in contact with the other.  The effort of the armature to touch the pole it nearly touches places the diaphragm under tension.  The free arm of the magnet is surrounded by a coil 4, whose ends extend to form the line.

[Illustration:  Fig. 1.  Type of Magneto Telephone]

When sound vibrates the diaphragm, it vibrates the armature also, increasing and decreasing the distance from the free pole of the magnet.  The lines of force threading the coil 4 are varied as the gap between the magnet and the armature is varied.

The result of varying the lines of force through the turns of the coil is to produce an electromotive force in them, and if a closed path is provided by the line, a current will flow.  This current is an alternating one having a frequency the same as the sound causing it.  As in speech the frequencies vary constantly, many pitches constituting even a single spoken word, so the alternating voice currents are of great varying complexity, and every fundamental frequency has its harmonics superposed.