CHAPTER VIII. THE TELEPHONE TRANSMITTER AND RECEIVER. THE PHOTO PHONE. THE THERMOPHONE. THE SELENIUM CELL. THE SPEAKING ARC.
The telephone is an instrument for the transmission of sounds to a distance by the agency of electricity, wherein the speaker talks to an elastic plate of thin sheet-iron, which vibrates and transmits its every movement, electrically, causing it to vibrate in an identical manner and emit the same sounds.
The transmission of the vibrations depends upon well known principles of electricity, and consists, not of an actual transmission of the sounds, but the passage of electric waves, or impulses, which keep perfect accord and agree in phase and period with the atmospheric waves produced by the voice. These in turn, through the medium of an electromagnet, cause vibrations of a plate or membrane, which agitates the air in a manner similar to the original disturbance, and thus emits sounds.
The parts of the apparatus which take up the sound waves and change them into electric currents compose the transmitter. In the form of transmitter most commonly used, the motions of the diaphragm cause variations in the strength of a current flowing from a battery by varying the resistance in the path of the electric current.
The sounds are directed to the mouthpiece, which causes the vibrations of the air to strike the diaphragm, on the back and center of which is fastened a small cup shaped piece of carbon. A second cup is mounted in a rigid position directly in back of the first. The space between is filled with small polished granules of carbon,
FIG. 131.—Diagram of a telephone transmitter.
When these are in a perfectly free and loose state their resistance to an electric current is very great, and they allow almost none to flow.
When slightly compressed their resistance is greatly lowered, and they permit the current to pass. The vibrations of the diaphragm exert a varying pressure upon the granules, with a corresponding variation in their resistance and the amount of current flowing.
