transmitted over the wire, or transmission line, and enter the receiver of the listener's telephone set. The receiver converts
the electrical waves back into sound waves which, again, correspond in waveform and frequency to the original sound
waves. The listener in his receiver thus hears words corresponding to those spoken into the distant transmitter.
(2) This process is shown in block form in figure 8. Above, on each side, is a graph of the sound waves as
spoken and heard. The electrical wave is shown in the center.
(3) The fundamental principle of the telephone can be summarized by the explanation that electrical waves,
traveling over wires, are substituted for sound waves, traveling in air, over the major portion of the distance separating the
speaker and listener. Various types of telephone systems are in use, but this underlying principle is common to them all.
a. Function of telephone transmitter. The function of the telephone transmitter is to convert waves of sound into
waves of electric current of corresponding waveform and frequency. The energy of the waves of electric current so
generated must travel over wires for relatively long distances, and arrive at the receiver at a level providing normal
listening. But energy is lost in transmission over wires. Because of this loss, the initial energy of the electrical waves
must be made greater than the original energy of the sound waves. The circuit of the transmitter therefore must provide a
means of supplying this extra energy to the electric waves which it generates.
FIGURE 8. Transmission of Sound by