COMMUNICATION BY TELEGRAPH 



271 



The application of scientific principles has made it 

 possible for telegraph, companies to save millions of 

 dollars in the construction of lines and equipment. By 

 using certain methods and certain instruments it is 

 possible to send two, four, or eight messages over a 

 single wire at the same time. A system in which two 

 messages are sent at the same time is called a duplex 

 and one in which four messages are sent is a quadru- 

 ple* system. 



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 

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



12 

 Van Buskirk and Smith, The Science of Everyday Life, Chap. 



18 



Webb and Beauchamp, Science by Observation and Experi- 

 ment, Unit 2 

 Wood and Carpenter, Our Environment: How We Use and 



Control It, Topic 12 



Special references 



Meister, Magnetism and Electricity 



Morgan, The Boy Electrician 



Seaver, The American Boy's Book of Electricity 



This central copper wire carries the electric current. 



If it breaks, these flexible copper tapes carry the cur- 

 rent around the gap. 



This is the permalloy tape whose magnetic qualities 

 give the cable its great speed. 



A thick covering of gutta-percha holds the currents 

 to their path. 



A wrapping of jute cushions the pressure of several 

 miles of sea water. 



Eighteen steel armor wires protect the cable from 

 injury. 



The outer coat is a wrapping of tarred hemp cords. 



Courtesy Western Union 



FIG. 438. NEW TYPE OF CABLE 



WHAT YOU SHOULD AIM TO ACQUIRE FROM 

 THIS STUDY 



1. Know the construction and operating principle of 

 the telegraph key, sounder, and relay. 



2. Acquire skill in connecting these devices into a 

 complete telegraph circuit. 



3. Know something of modern telegraphy and how it 

 is carried on. 



4. Know how telegraph messages are sent across the 

 ocean by cable. 



TEST OF MASTERY OF THE TOPIC 



In your notebook complete the statements, answer the 

 questions, and comply with the instructions. 



1. Explain briefly how an electromagnet differs from a 

 permanent magnet. 



2. The key in a tele- / A 

 graph circuit is used to 



3. The device shown in 

 Figure 439 is called 



4. The part marked A 



is made of because it 



must be a substance. 



5. The are shown FIG. 439 



at C. 



6. The bar marked B is made of because it is a . 



metal. 



7. The device in Figure 440 is called a 



and is used 



to . 



8. What is A and what does it do when the device is op- 

 erating? 



9. Make a sketch drawing showing a key, sounder, and 

 battery at each end of a telegraph line which uses only one 

 wire. 



10. Make a sketch draw- 

 ing of a relay and local 

 circuit showing how the 

 connections are made. 



11. Can more than one 

 message be sent over one 



wire at a time ? Explain. FIG. 440 



12. Does the relay 



strengthen the electric current which comes to it? Explain. 



13. Explain how the relay differs in construction and use 

 from the telegraph sounder. 



14. What are some of the modern devices used by tele- 

 graph companies? 



15. In making a home telegraph set a boy decided that 

 he could prevent magnetic loss in his set and therefore 

 make the magnet stronger by winding insulated iron wire 

 on a brass bolt and by using brass parts throughout. Criticise 

 his method and tell how he should have built the set. 



16. In a commercial telegraph circuit the is used 



as a return circuit. 



17. Suggest the reason why the switch on the telegraph 

 key must be open when messages are being sent from that 

 instrument. 



18. Where does the energy come from that operates the 

 sounder in the local circuit of a telegraph system having a 

 relay ? 



19. Why must code signals be used in sending telegraph 

 messages ? 



20. Where does the energy come from that operates the 

 relay in a telegraph circuit? 



