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COMMUNICATION 



used to produce electrical disturbances in the ether. 

 When electrons surge back and forth in a wireless 

 circuit, they are said to oscillate. 



What are wireless waves? Our study thus far has 

 shown that ether waves may be set up by causing 

 electrons to surge back and forth. This may be ac- 

 complished by a spark coil, an electric generator, or 

 a vacuum tube. The waves which are set up as a re- 

 sult of surging electrons are like light waves, heat 

 waves, and ultra-violet waves which cause sunburn. 

 All of these waves travel through the ether at the 

 tremendous speed of 186,264 miles per second. This 

 is about seven and a half times around the earth in 

 one second. Also wireless waves are different from 

 light waves, heat waves, and ultra-violet waves. To 

 understand how they are different you must learn 

 a little more about waves. 



The distance from the top or crest of one wave to 

 the top of the next one is called a wave length. You will 

 recall from your study of sound, in the unit on air, 

 that musical tones are varied in pitch by varying the 

 rate of vibration or frequency of the sounding body 

 and also that while sound waves may all travel at the 

 same rate they may differ in wave length and fre- 

 quency. Tones of high pitch or frequency have short 

 wave lengths and tones of low frequency correspond- 

 ingly longer wave lengths. 



These principles all apply to a knowledge of light 

 waves, heat waves, ultra-violet waves, wireless and 

 radio waves. While all of these waves travel at the 

 same speed as light, they are all different in wave 

 length and frequency of vibration. Ultra-violet light 

 has a wave length too short to be seen by the human 

 eye, while heat waves, wireless waves, and radio waves 

 are too long to be seen by the eye. The ultra-violet 

 waves have a short length but a high vibration fre- 

 quency, while the longer radio waves have lower vi- 

 bration frequencies. 



FIG. 459. DIAGRAM OF A CRYSTAL RECEIVING SET 



How are electric waves and messages received 

 without wires? About the time that Hertz was experi- 

 menting in Germany with the production of electric 

 waves, Professor Branly, a Frenchman, discovered 

 how these same waves might be used to ring a bell 

 in another room. He made the discovery while ex- 

 perimenting with metal particles in a glass tube, try- 

 ing to get electricity through them. A spark coil near 

 by was started, and to Branly's surprise the metal 

 particles were affected in such a way that current 

 flowed through them. By placing a tube of metal par- 

 ticles in an electric bell circuit with batteries and 

 starting a spark coil in another room, he could ring the 

 bell. 



Marconi, an Italian inventor, made improvements 

 on the Branly "coherer," as it was called, and used it 

 in his experiments first in England and later, in 1901, 

 when wireless messages were first sent across the 

 Atlantic Ocean. 



The coherer as a device to detect wireless and ra- 

 dio waves was displaced when it was discovered that 

 certain crystals such as galena, a crystal made up 

 of lead and sulphur, could be used. We shall study 

 how such a crystal detects the wireless wave. Figure 

 459 shows how the crystal is connected to the aerial, 

 ear phones, and ground to make a simple receiving 

 set. 



The surging electrons in the secondary circuit of 

 the spark coil send ether waves out into space from 

 the aerial. These waves are vibrating very rapidly, or 

 as we have learned to say, have a high frequency, 

 much too high to be heard by the human ear or to be 

 seen by the eye. 



As these waves encounter a receiving aerial they 

 set the electrons surging in its wires just as they 

 surged in the sending aerial, first in one direction and 

 then in the other. 



The crystal detector acts as an electron valve. 

 When the electrons go in one direction they can get 

 through the crystal, but when they are moving in the 

 opposite direction they cannot get through to the 

 ground ; hence they must go around through the ear 

 phones and into the ground. Study Figure 460. 



The electrons that must go through the ear phones 

 to get to the ground form a current in the telephone 

 electromagnets and cause the little iron disk to vi- 

 brate, sounding the dots and dashes which were sent 

 out at the sending station. 



While the crystal detector is still used for receiving 

 wireless waves, it has been largely displaced by the 

 vacuum tube. By use of the vacuum tube not only 

 are electron surges detected, but they may be ampli- 

 fied hundreds of times. The next topic on radio will 

 tell more about this invention and how it works. 



