272 



COMMUNICATION 



TOPIC 3. THE TELEPHONE IN COMMUNICATION 



SUGGESTED PROBLEMS AND QUESTIONS 



1. What is the early history of the telephone? 



2. How does the electric bell work? 



3. How does the telephone transmitter work? 



4. How does the telephone receiver work? 



5. What happens at the telephone central? 



SUGGESTIONS AND HELPS FOR STUDY 



1. Become thoroughly familiar with each problem 

 as stated before attempting any reference study or in- 

 vestigation of it. 



2. A trip to a telephone central will serve to help 

 you understand many of the interesting things which 

 go on there and will also teach you many of the prob- 

 lems confronted in such an office. 



3. The study of the operation of a dial telephone is 

 very interesting. It will teach you how man has been 

 able to reduce the labor of human hands through elec- 

 tricity. 



4. You may find the following new words and 

 phrases in this topic : 



compress to squeeze together. 



diaphragm a disk which vibrates. 



grounded circuit a pathway for electricity where the 

 ground is used instead of a return wire. 



vibration a to-and-fro movement like that of a pendu- 

 lum. 



EXPERIMENTS OR DEMONSTRATIONS WHICH WILL 

 HELP ANSWER THE PROBLEM QUESTIONS 1 



Experiment 164. How is the telephone transmitter 

 constructed and how does it work? 



Secure two pieces of copper or other metal about four 

 inches square. Punch a hole near the edge of each piece 

 with a nail and attach a two-foot length of bell wire as 

 shown in Figure 441. Break the carbon rod from an old 



RECEIVER 



GRANULES BETWEEN 



FIG. 441 



dry cell into pieces about the size of a small pea. Pick out 

 a handful of pieces that are uniform in size to use in the 

 transmitter which you are making. 



Place one of the copper plates on the glass face of an 

 alarm clock and lay the carbon particles around on the 

 plate. Place the other copper plate over the carbon, being 

 careful that the plates touch neither the metal part of 

 the clock nor each other. Connect the wires from the plates 

 through a telephone receiver and one or two dry cells. 

 What do you hear in the receiver? 



Study the cross-section diagram of a telephone trans- 

 mitter in Figure 442 and compare its parts with those of the 

 one which you have built. 



Remove the rubber mouthpiece from a telephone trans- 

 mitter and while holding your ear close, press the ex- 



1 See workbook, p. 97. 



Courtesy Bell Telephone Laboratories 



FIG. 442. CROSS SECTION OF TRANSMITTER 



posed diaphragm lightly with the rubber eraser of a pen- 

 cil. Can you hear the grating of the carbon particles? 



In your notebook record your notes and list the conclu- 

 sions which might be drawn from the data of this experi- 

 ment. 



Experiment 165. How is the telephone receiver con- 

 structed and how does it work? 



Carefully unscrew the rubber earpiece on the end of the 

 receiver shell. Find the diaphragm made of thin metal 

 and remove it. If possible slide the shell back on the lead 

 wires and expose the inside of the receiver. Let these ques- 

 tions guide your observation. How many wires lead into 

 the receiver? Where do they appear to go? What appears 

 to be on the earpiece end of the receiver close to the metal 

 disk? How many of these are there? What seems to make 

 up the back part of the receiver? Test it by bringing an iron 

 tack near it. Bring the metal disk to the place from which 

 you first removed it. What holds it in position? Can you 

 suggest what might make this vibrate ? 



In a short paragraph clearly summarize what you have 

 learned from this experiment. 



Devise an experiment to test whether a telephone receiver 

 can be used as a transmitter. Test your experiment. 



Experiment 166. How may a telephone line be set up? 



The diagram (Fig. 443) suggests two ways which you can 

 use in the laboratory. Set up both if your time will permit. 



DRY CELLS 



RECEIVER 



TRANSMITTER 



TRANSMITTER 

 AND RECEIVER 



DRYCE.US 



FIG. 443 



TRANSMITTER 

 AND RECEIVER 



