TELEGRAPHING WITHOUT WIRES 



277 



2. How the telephone transmitter works. 



3. How the telephone receiver works. 



4. What happens at a telephone central. 



5. An attitude toward the importance of the telephone 

 in everyday life. 



TEST OF MASTERY OF THE TOPIC 



In your notebook complete the statements, answer the 

 questions, and comply with the instructions. 



1. Explain in detail how a sound wave is changed into 

 an electric current by the telephone transmitter. 



Courtesy Bell Telephone Laboratories 

 FIG. 454 



2. Some of the parts in the cross section of a tele- 

 phone transmitter (Fig. 452) are lettered. In your note- 

 book indicate the part which each letter represents. 



3. The parts of the electric bell in Figure 453 are let- 

 tered. Name the part which each letter represents. 



4. travel over the wire in a telephone system. 



5. invented the electromagnet. 



6. invented the telephone in . 



7. In the cross section of the telephone receiver in Fig- 

 ure 454, some of the parts (electromagnet, permanent mag- 

 net, shell, and diaphragm) are lettered. Indicate the part 

 which each letter represents. 



8. Explain the importance of each of the following parts 

 in the receiver: permanent magnet, electromagnet, disk. 



9. In the telephone transmitter the disk is made of 

 aluminum, while in the receiver it is a thin magnetic alloy. 

 How do you account for this? 



10. Write a paragraph on "What takes place at the tele- 

 phone central." 



11. What sort of current is used in a telephone system to 

 carry the messages ? 



12. What kind of current is used to ring the bell? 



13. Would it be possible to make use of a bar or horse- 

 shoe magnet in an electric bell? Explain. 



14. A sound wave is made up of a series of and . 



15. Write a summary paragraph pointing out the impor- 

 tance of telephone communication in modern life. 



TOPIC 4. TELEGRAPHING WITHOUT WIRES 



SUGGESTED PROBLEMS AND QUESTIONS 



1. What common experiences have you had with 

 waves? 



2. How are wireless waves set up and messages 

 sent? 



3. How are electric waves and messages re- 

 ceived without wires? 



SUGGESTIONS AND HELPS FOR STUDY 



1. This topic deals with sending telegraph code 

 messages without the aid of wires. In this respect it 

 should not be confused with radio, where the human 

 voice is sent without wires. 



2. Study the problems and see if you are interested 

 in them or if they are worth while to you. 



3. In handling the induction coil of the sending set 

 while it is turned on, be very careful not to touch the 

 wires leading to the aerial or the ground, as a severe 

 electric shock will result. 



4. In the study of this exercise you may find the 

 following new words and phrases : 



aerial the part of a radio receiving or broadcasting set 

 which is usually elevated in the air to send out or re- 

 ceive radio waves. 



ether a medium assumed by science to carry light, heat, 

 and radio waves. It is assumed to fill all space, even 

 between the molecules of matter. 



wave length the distance from the top of one wave to 

 the top of the next one. 



cohere to cling together. 



electromagnetic waves waves which travel through the 

 ether. 



frequency the number of vibrations per second which 

 the ether wave is making. 



induction coil a device which changes a low voltage di- 

 rect current into a high voltage current. 



primary circuit the circuit on the induction coil which 

 contains the batteries. 



secondary circuit tKe high voltage circuit of the induc- 

 tion coil. 



EXPERIMENTS OR DEMONSTRATIONS WHICH WILL 

 HELP ANSWER THE PROBLEM QUESTIONS 1 



Experiment 168. How may wireless messages be sent? 



Caution: The high voltage of the spark coil is dangerous. 

 Be careful. 



A spark coil is needed for this experiment. This can be 

 secured from an automobile (some types) or from any 

 mail-order or scientific apparatus house. The key used in 

 the experiment on the telegraph may also be used here. 



Connect the key, a knife switch, and batteries into the 

 circuit as illustrated in Figure 455. Be sure that the switch 

 is open. Lead a wire from each of the secondary or high 

 voltage binding posts to make a spark gap of about a 

 quarter inch. Also lead a wire from one secondary binding 

 post to the aerial and from the other one to a ground 

 connection such as a water pipe or radiator. 



The vibrator on the coil may need adjustment; if so, 



1 See workbook, p. 100. 



