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 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]