b. Telephone transmitters.
(1) Paragraph 17a(1) describes the principle of operation of the earliest type of instrument used as a transmitter.
This type of transmitter had a coiled wire wound around one pole of a permanent magnet, and a thin metal wafer of
magnetic material, called the diaphragm, mounted adjacent and at right angles to the magnet. Sound waves colliding with
the diaphragm would cause it to vibrate at a frequency determined by the frequency of the condensations and rarefactions
of the air molecules, as illustrated in figure 3. It will be understood that the intensity of these condensations and
rarefactions vary with each change in characteristic among the various sound waves, and that the amplitude of each
diaphragm motion also will be affected by the same conditions; accordingly, the frequency and the amplitude of the
diaphragm vibrations will cause the density of the magnetic field, in which it is located, to change with each change of
position of the diaphragm. Since this varying magnetic field is cutting across the coiled wire, a voltage is induced in the
wire. The voltage thus induced is alternating, since all induced voltages are alternating voltages.
(2) If two wires now are connected to the coiled wire ends and their extremities are connected in turn to another
instrument similar in construction to that described (1) above, the induced ac voltage will cause a variation in the strength
of the associated permanent magnet, and, since the strength of the permanent magnet field determines the instantaneous
position of the diaphragm, each change in current intensity and direction of flow will cause a change in the position of the
diaphragm. Because these changes are at the same frequency as those of the diaphragm at the originating point, the
diaphragm at the terminating point will reproduce the same waves established originally.
(3) This entire process encompasses the changing of sound energy into electric energy, transmitting the signals
electrically and then reconverting the electric energy into sound energy.
(4) The distance of which this process can be applied usefully is quite limited, since no provisions are made
for amplifying the original energy provided by the sound waves present at the originating end. If all of this energy
could be reserved for operating the diaphragm at the distant end, the distance between telephones could be
extended almost indefinitely; however, this cannot be so, because part of the original energy is used in over coming