COMMUNICATION BY TELEGRAPH 



269 



click which can be easily heard. On some sounders 

 part of the bar is made of aluminum to make it light. 

 This moving bar is called the armature. 



The simple telegraph shown in Figure 431 has a 

 complete circuit of wire. If a single wire were strung 

 from one room to another, a key, sounder, and battery 

 might be connected as shown in Figure 432. Notice 

 that a wire from the key and one from the sounder 

 are connected to a water pipe. In this way the elec- 

 trical circuit is completed through the metal water 

 pipes of the building. Connection may be made to 

 the pipes of the heating system. In long distance 

 telegraphy the earth is used as a return, and only 

 one wire needs to be used. This makes for a great sav- 

 ing in cost of wire and line construction. 



If a key and a sounder are to be placed at each end 

 of the circuit it may be arranged as shown in Figure 

 428. The water pipe is again used as a return. When 

 such a system as this is assembled it is evident that 

 each key makes an opening in the circuit and no mes- 

 sage can be sent unless one of the keys is held down 

 while the other is used in sending a message. This 

 might prove difficult; hence, all telegraph keys have 

 a switch which is closed when a message is being 

 received but open when the key is being used for send- 

 ing. 



How are messages sent long distances on land and 

 under sea by telegraph? In your study of the unit on 



electricity you learned 

 that long wires give 

 more resistance to the 

 flow of an electric cur- 

 rent than shorter ones 

 of the same size. 



When a telegraph 

 line is more than 350 

 miles in length, the cur- 

 rent from batteries or 

 even dynamos is too 

 feeble to operate the 

 sounder because of the 

 resistance. This diffi- 

 culty is overcome by 

 using a device called a 

 relay. Figure 433 shows 

 a relay, and how it is 

 connected in a tele- 

 graph line. 



The relay is a com- 

 bined instrument, part 



of it working like a very sensitive sounder and part 

 like a key. Let us study its construction and work- 

 ings. The sounder-like part of the instrument is made 

 up of two electromagnets which are wound with many 

 turns of fine wire. The feeble line currents, too weak 



Sounder 



H'H 



FIG. 433. A RELAY AND HOW IT IS 

 CONNECTED 



to operate a sounder, are passed through these coils. 

 The feeble currents going through so many coils 

 make sufficient magnetic force to attract the arma- 

 ture or moving part. How does the armature of the 

 relay seem to compare in weight with that of a 



LOS ANGELES 



NEW YORK 



SOUNDER 



SOUNDER 



LOCAL 



LINf BFTWEEN 

 STATIONS 



RELAY-" 



KE.Y 



BATTERY--" 



Ci','\\ .SWITCH 

 / \i* (OPEN) 



lll|l|l I l|h 



G- ROUND 



BATTERY 

 G-ROUND 



FIG. 434. COMPLETE TELEGRAPH SYSTEM 



sounder? Would this help to make the instrument 

 more sensitive than a sounder? The armature of the 

 relay, unlike that of the sounder, is not constructed 

 to make clicks but to act as a key. The armature of 

 the relay is connected into another electrical circuit 

 (see Figure 433) with a battery and sounder. The 

 feeble line currents do not reach this circuit, for 

 they pass around the relay electromagnet from the 

 line and then out to the return wire or ground. 



As the armature of the relay is pulled up by the 

 electromagnets, it closes the little local circuit just 

 as a key would. This permits current from the local 

 battery to flow and operate the sounder. The arma- 

 ture of the sounder works exactly with the arma- 

 ture of the relay, making dots and dashes as they 

 come over the line wire. Figure 434 shows a diagram 

 of a complete telegraph system with key sounders, re- 

 lays, and batteries between two large cities. Study the 

 diagram carefully and see if you can answer the ques- 

 tions. 



At which end of the line is the message being sent? 

 How can you tell? How does the current get back for 

 a complete circuit when there is only one line wire? 

 What changes would be made if the operator at the 

 end of the line sending wished to receive a message 

 from the other city? When New York is sending, can 

 the operator hear his own message as he sends it? 

 Does the line current get into the local circuits? Ex- 

 plain your answer. 



If you were to go to a modern commercial tele- 

 graph office you no doubt would be surprised to see 

 how the messages are actually sent and received. The 

 older style key and sounder have been replaced by 

 automatic devices which make it possible to send and 



