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
inward stroke of the diaphragm and a second half-cycle of alternating
current by the succeeding outward stroke, these half-cycles flowing in
opposite directions. Assume one complete cycle of current to pass
through the line and also through another such device as in Fig. 1 and
that the first half-cycle is of such direction as to increase the
permanent magnetism of the core. The effort of this increase is to
narrow the gap between the armature and pole piece. The diaphragm will
throb inward during the half-cycle of current. The succeeding
half-cycle being of opposite direction will tend to oppose the
magnetism of the core. In practice, the flow of opposing current never
would be great enough wholly to nullify and reverse the magnetism of
the core, so that the opposition results in a mere decrease, causing
the armature's gap to increase and the diaphragm to respond by an
outward blow.

Complete Cycle of Conversion. The cycle of actions thus is complete;
one complete sound-wave in air has produced a cycle of motion in a
diaphragm, a cycle of current in a line, a cycle of magnetic change in