318 



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 



^////////7/////m//, 



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 



