d 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
a core, a cycle of motion in another diaphragm, and a resulting wave
of sound. It is to be observed that the chain of operation involves
the expenditure of energy only by the speaker, the only function of
any of the parts being that of _translating_ this energy from one form
to another. In every stage of these translations, there are losses;
the devising of means of limiting these losses as greatly as possible
is a problem of telephone engineering.

[Illustration: Fig. 2. Magneto Telephones and Line]

Magneto Telephones. The device in Fig. 1 is a practical magneto
receiver and transmitter. It is chosen as best picturing the idea to
be proposed. Fig. 2 illustrates a pair of magneto telephones of the
early Bell type; _1-1_ are diaphragms; _2-2_ are permanent magnets
with a free end of each brought as near as possible, without touching,
to the diaphragm. Each magnet bears on its end nearest the diaphragm a
winding of fine wire, the two ends of each of these windings being
joined by means of a two-wire line. All that has been said concerning
Fig. 1 is true also of the electrical and magnetic actions of the
devices of Fig. 2. In the latter, the flux which threads the fine wire
winding is disturbed by motions of the transmitting diaphragm. This
disturbance of the flux creates electromotive forces in those
windings. Similarly, a variation of the