282 



COMMUNICATION 



up in the receiving aerial by the ether waves from the send- 

 ing station. 



13. The current in the receiving aerial is an current. 



14. What type of current is used in a telephone receiver? 



15. The function of the crystal is 



16. In your notebook connect by pencil line the parts 

 of a simple wireless receiving set like the one in Figure 

 462. 



17. The colors of the visible spectrum in the descending 

 order of their wave lengths are 



18. When electrons surge back and forth rapidly in a 

 wireless circuit they are said to be 



19. Sound waves are detected by the ; light waves, 



by the ; and wireless waves, by means of a which 



permits 



to flow through it in one direction. 



20. An example of ether waves shorter than those of the 

 visible spectrum is . 



21. An example of ether waves longer than those of the 

 visible spectrum is 



TOPIC 5. COMMUNICATION BY RADIO 



SUGGESTED PROBLEMS AND QUESTIONS 



1. How are wireless and radio alike? 



2. What is a vacuum tube and how does it 

 work? 



3. What happens at the broadcasting station? 



4. How does a radio receiving set work? 



5. What are some of the recent advances in the 

 application of the principles of the radio ? 



SUGGESTIONS AND HELPS FOR STUDY 



1. Secure a used vacuum tube which can be taken 

 apart for study. If you are careful, the glass bulb can 

 be filed around the brass base collar and easily re- 

 moved, showing the inside of the tube clearly. 



2. If you live near a broadcasting station, make a 

 visit to it and study the equipment carefully. 



3. Study the inside of a radio set either at home or 

 at school to learn the number of tubes used, the num- 

 ber and type of transformers, the type of coils, and 

 the number and type of condensers. 



4. In the study of this topic you are likely to run 

 into ideas and readings which you will not clearly 

 understand. Do not be discouraged, for the science of 

 radio has many difficult things in it. The explanation 

 of tuning a radio set by condensers and coils involves 

 a great deal of physics which will be difficult for you 

 until you have studied more about electricity. 



5. The following new words and phrases may be 

 found in this exercise : 



"A" battery a low voltage battery used to supply en- 

 ergy to the filament of the vacuum tube. 



"B" battery a high voltage battery which keeps the 

 plate of the vacuum tube charged positively. 



antenna the wires which are put out, usually at some 

 height, to receive or send radio messages. 



audion a special name applied to the vacuum tube. 



condenser a device used in tuning a radio circuit ; it 

 is made up of successive layers of metal plates sepa- 

 rated by layers of air or some other insulating ma- 

 terial. 



filament the fine wire in a vacuum tube which glows red 

 when supplied with electric current. 



grid the fine spiral of wire which surrounds the fila- 

 ment in the vacuum tube. 



frequency the number of vibrations per second. 



kilocycle a unit for measuring the frequency of radio 

 waves. One kilocycle equals 1,000 cycles. 



voice wave the varying electric current set up in the 

 transmitter or microphone circuit as a result of the 

 sound waves' entering the microphone ; the type of 

 electrical current which goes over the wire in tele- 

 phone communication. 



carrier wave the high frequency, alternating current 

 wave, which carries the voice wave from the broad- 

 casting station to the receiving aerial. 



FIG. 463 



modulated wave the carrier wave upon which the 



voice wave has been impressed at the broadcasting 



station. 

 audio-frequency vibrations which can be heard by the 



human ear. 

 radio-frequency vibrations which are above the range 



of the human ear, usually more than 100,000 cycles 



per second. 



EXPERIMENTS OR DEMONSTRATIONS WHICH WILL 

 HELP ANSWER THE PROBLEM QUESTIONS 1 



Experiment 170. How is the vacuum tube constructed? 



Carefully remove the glass bulb from an old vacuum tube 

 by filing around it next to the brass base with a sharp tri- 



1 See workbook, p. 101. 



