Radio Broadcast (May-Oct 1922)

RADIO BROADCAST 155 just as a telephone line is controlled by the telephone company. If there were no control, there would be no radio communication, for a great many people in the same vicinity would try to use the same wave length at the same time, with the same result, so to speak, that would occur if a great many people tried to use the same telephone line at the same time. SOUND WAVES AND THE VOICE SOUND waves are waves in air. The air is alternately compressed and rarefied; the compression corresponding to a crest of the wave, the rarefaction to the trough of the wave. A simple sound is made up of only one of these waves. More complex sounds are composed of a number of these waves. The voice, for instance, is a complex sound having, for a man, one wave whose frequency is about 250 per second, another of 500 per second, another of 750 per second, and so on. The wave of the lowest frequency is called the fundamental; the other waves are called harmonics or overtones. It is the number of harmonics present and their relative amplitude (strength) which make it possible to distinguish one voice from another. WIRE TELEPHONY IN ORDINARY wire telephony the sound waves produced by the voice are caused to produce, by means of a transmitter, a variation in a direct current; the variation in the current being identically similar in amplitude and frequency to the sound waves which produce it. This variation in direct current is usually converted, by means of a transformer, into a variation in alternating current which is similar to the variation in direct current. The variation in alternating current is then by means of a receiver converted into sound waves, the sound waves being identically similar in amplitude and frequency to the alternating current which causes them. As this identical similarity of amplitude and frequency has been maintained throughout the complete cycle, the sound waves produced by the receiver are identical with those originally produced by the voice. The series of events outlined above are represented by the curves of figure 4. The sound wave is represented in A of figure 4 in the same way as a radio wave is represented in figure 3. Tach crest of the wavy line represents a compression of the air particles and each trough represents a rare faction of the air particles. It is noted that the sound wave is a complex one. The instruments peculiar to wire telephony are the transmitter and the receiver. The transmitter, sometimes called a microphone, has two conductors separated by granules of carbon. The sound waves strike a flat piece of metal called a diaphragm and cause it to vibrate. The diaphragm acts upon the carbon granules, alternately increasing and decreasing the pressure of the granules upon one another, as it vibrates to and fro. This variation in pressure between the carbon granules varies the resistance of the granules. A direct current which is flowing through the granules is varied by this varying resistance. This varying direct current is changed into a varying alternating current by means of a step-up transformer. The alternating current acts upon the receiver. This receiver consists of an electromagnet through which the alternating current passes, and a permanent magnet which forms the core of the electromagnet. Mounted in front of the poles of this combination magnet is a flat piece of metal containing iron. This is also called a diaphragm. The alternating current causes the diaphragm to vibrate, thus producing the sound made at the transmitter. (It is realized that the above paragraphs contain some electrical terms with which the reader may not be familiar. These will be explained in later articles.) FUNDAMENTAL METHOD OF RADIO TELEPHONY IN RADIO telephony methods are employed to produce at the transmitter and reproduce at the receiver a sound wave, that is, a ne. 5.