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 electromotive forces in the
windings varies the pull of the permanent magnet of the receiving
instrument upon its diaphragm.

[Illustration: No. 10 SERIES MULTIPLE SWITCHBOARD _Monarch Telephone
Mfg. Co._]

[Illustration: Fig. 3. Magneto Telephones without Permanent Magnets]