. 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.

Conversion from Voice Currents to Vibration. The best knowledge of
the action of such a telephone as is shown in Fig. 1 leads to the
conclusion that a half-cycle of alternating current is produced by an
inwar