RADIO 



289 



lights and shadows in the scene vary in intensity. 

 These electrical waves from the photo-electric cell are 

 then amplified and sent out either by wire or wireless. 



At the receiving end the electrical waves are very 

 weak and so must be amplified again by means of 

 vacuum tubes. 



After amplification the waves are sent to a neon- 

 glow tube. This tube flashes as the amplified waves 

 are sent through it. The flashes vary just as the elec- 

 trical pulses varied with the lights and shadows in 

 the scene that was sent. When these flashes are viewed 



Courtesy Bell Telephone Laboratories 



FIG. 481. TELEVISION RECEIVING DEVICE 



through a scanning disk, which is like the one in the 

 sending set and which is turning at the same speed, 

 the scene that was sent out is recreated for an ob- 

 server looking into a small frame as shown in Figure 

 481. 



Recently improved methods of scanning have been 

 developed, one of which makes use of another type of 

 tube known as a cathode ray tube. 



REFERENCES FOR FURTHER STUDY 



Texts 



Caldwell and Curtis, Science for Today, Unit 9 



Clement, Collister, and Thurston, Our Surroundings, Chap. 



17 (part) 

 Hunter and Whitman, Science in Our World of Progress, 



Unit 10 

 Lake, Harley, and Welton, Exploring the World of Science, 



Chap. 25 

 Pieper and Beauchamp, Everyday Problems in Science, Unit 



16 



Powers, Neuner, and Bruner, Man's Control of His Environ- 

 ment, Chap. 23 

 Skilling, Tours through the World of Science, Tours 11 and 



12 

 Van Buskirk and Smith, The Science of Everyday Life, 



Chap. 18 

 \Vatkins and Bedell, General Science for Today, Chap. 25 



Webb and Beauchamp, Science by Observation and Ex- 

 periment, Unit 2 



Wood and Carpenter, Our Environment: How We Use and 

 Control It, Topic 12 



V 

 Special references 



Lunt, Everyday Electricity 



Meister, Magnetism and Electricity 



Morgan, The Boy Electrician 



Seaver, The American Boy's Book of Electricity 



Mills, The Letters of a Radio-Engineer to His Son 



Kendall and Koehler, Radio Simplified 



Darrow, Masters of Science and Invention 



American Telephone and Telegraph Co., Through Electrical 

 Eyes 



American Telephone and Telegraph Co., Synchronized Re- 

 production of Sound and Scene 



American Telephone and Telegraph Co., Two-way Television 



WHAT YOU SHOULD AIM TO ACQUIRE FROM 

 THIS STUDY 



1. A knowledge of the construction of the vacuum 

 tube. 



2. How the vacuum tube acts as a detector of radio 

 waves. 



3. How radio frequency waves are modulated by voice 

 waves. 



4. A knowledge of the meaning of tuning and some- 

 thing of how it is accomplished in a radio set. 



5. A knowledge of the parts used in a radio receiving 

 set. 



6. A knowledge of what happens in a broadcasting 

 set. 



7. The concept of radio waves and how they are trans- 

 mitted. 



8. An understanding of some of the recent advances 

 in communication. 



TEST OF MASTERY OF THE TOPIC 



In your notebook complete the statements, answer the 

 questions, and comply with the instructions. 



1. The radio waves sent out by different broadcasting 



stations are alike in that they travel at the but different 



in . and 



2. The high frequency radio wave is sometimes called 

 the wave. 



3. In general wave length transmission is used for 



sending long distances. 



4. When the electric wave from the microphone is im- 

 pressed on the high frequency wave, the latter is said 

 to be a wave. 



5. The three essential parts of the vacuum tube are _ 



6. The principal use of the hot is to furnish elec- 

 trons. 



7. The is always positively charged. 



8. The aerial currents come to the of the vacuum 



tube. 



9. A radio receiving set will pick up a given station only 

 when it is ___ to that station's , 



10. Vibrations which are above the limit of the human 

 ear are often spoken of as 



11. When a vacuum tube acts as a detector it is essen- 



