Page:Popular Science Monthly Volume 92.djvu/334

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��Popular Science Monthly

��can in some way set the air, ether or other transmitting medium into vibration; the second is an instrument which is capable of indicating the presence of the vibrations. In speaking, the human apparatus con- sisting of the mouth and vocal system produces sound-waves, or compressional vibrations, in the air. The sound waves vary in intensity and pitch (or frequency) according to the signaling code with which we are all familiar and which we call a language. When the air vibrations reach the ear of the listener, they are re-converted into the sensation known as sound, and their presence is thus indicated.

The Basic Process of Radio Telegraphy

In radio telegraphy the process is identi- cal, from the broad viewpoint. As shown in Fig. 27, a sending apparatus indicated diagrammatically by the box marked S, is connected with an elevated aerial wire or antenna A and with the ground E. The sender S sets up high-frequency currents in the wire A, and the rushing of these charges up and down at the rate of hundreds of thousands of complete trips per second creates vibrations or waves in the ether, which surrounds the sender and extends indefinitely into space. The general form of these waves is shown by the dotted lines- in Fig. 1 and the way in which they pass off from the sender is indicated by the arrows. Just as the sound waves travel through the air and ordinarily produce no appreciable effects until they reach the receivmg ear, so do the radio waves pass through the ether. They are in- visible and inaudible, and produce no appreciable effects until

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Fig. 28 The waves as they reach the receiver

��they strike such a conducting body as an elevated aerial wire. When an antenna structure of this kind is reached, as shown in Fig. 28, the waves set up in it a series of small high-frequency voltage (or electrical pressure) impulses, first in one direction and then in the other. These minute voltages cause high frequency currents to flow in the antenna wire A, and these, passing rapidly

��to and from the ground connection. E through the receiving apparatus indicated by the box R, produce an indication wnich announces the arrival of the radio waves. If the waves are sent out in groups cor- responding to a pre-arranged code, or if their intensity or frequency is modified according to the code, messages may be transmitted. Thus, the object of the radio transmitter is to produce high-frequency currents in an elevated aerial wire system and to provide for the control of the current-production in accordance with some signaling code. This brings us to the first problem of radio- telegraphy, namely, the production of the high frequency currents. Before it can be considered we must reach a conclusion as to the numerical value of the frequencies involved.

Audio and Radio Frequencies

Frequency itself, in the abstract, is merely the number of times something happens in a given interval. The postman who delivers 400 letters in his eight-hour working day is evidently giving out mail at the rate of 50 letters per hour. If a water tank holding 8,000 gallons springs a leak and becomes empty in four hours, it is clear that the water is lost at the rate of 2,000 gallons per hour or SSJ^ gallons per minute. If a sound wave is produced by a siren which ejects 256 small puffs of air in each second, the wave consists of 256 compres- sions and rarefactions per second and corresponds to the tone of middle C on the musical scale.

Sound frequencies range roughly from 30 per second to 10,000 per second, and air vibrations which occur at rates either lower or higher than these respective extremes are ordinarily inaudible. Such frequencies as lie within these limits are called "audio frequencies," for convenience. They cor- respond to various musical tones, and to the electrical current frequencies which would produce those tones by passing through the ordinary magnetic telephone receiver. A frequency of 60 cycles per second, which defines an electric voltage or current alter- nating in direction completely 60 times per second, would produce a tone a little lower than that of the second C below middle C on the piano. Sixty cycles is a standard alternating current frequency; 500 cycles per second, in which the reversals occur over eight times as often, produces directly

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