TELEGRAPH 



5734 



TELEGRAPH 



armature of the magnet. This armature is 

 fastened to an arm of brass or wood. The arm 

 is pivoted and held up by a rubber band or 

 a spring made of brass or steel wire. The 

 sounder then appears as in Fig. 1. A key for 

 opening and closing the circuit can be made of 

 a piece of spring brass or spring steel, or an or- 

 dinary bell push button can be used. The key 

 will then appear about as in Fig. 2. Only one 

 line wire is needed. For the ground connec- 



ORDINART KEY AND SOUNDER 



tions connect the wires to gas or water pipes. 

 A key and sounder similar to that shown in the 

 above illustration may be purchased for less 

 than five dollars. 



The Ground. Connecting the ends of the line 

 to the earth makes it possible to use only one 

 wire, the earth taking the place of the return 

 wire. We may think of the earth as a great res- 

 ervoir of electricity. An electric charge which 

 goes along the line into the earth does not re- 

 turn to the other end of the line, but another 

 charge goes from the earth into the line. We 

 may compare such a current to a current of 

 water in a pipe laid along the shore of a lake. 

 Water is pumped in at one end of the pipe, 

 and it flows out at the other end into the lake. 

 The water which flows out of the pipe into the 

 lake does not return to the other end of the 

 pipe, nevertheless the lake furnishes a supply 

 of water so that the current flows as long as the 

 pump continues to work. In a similar way the 

 electric current in a single wire telegraph line 

 continues to flow as long as the battery or 

 dynamo continues to work. 



The Relay. For long-distance work the cur- 

 rent from a battery, or even from a dynamo, 

 is too feeble to operate the sounder. This is 

 true because of the resistance a long line wire 

 offers to the passage of the electric current. 

 The practical limit of the single telegraph line 

 in fair weather is about 450 miles; in rainy 

 weather the limit is from 150 to 250 miles. 

 However, messages are sent in all kinds of 

 weather over distances of thousands of miles. 

 This is accomplished by means of a relay, A 



relay is a contact key which is operated by 

 means of an electromagnet. It is operated in 

 the same way as a sounder. Suppose that when 

 the armature of a sounder is pulled down by 

 the magnet it brings two contacts together and 

 closes another circuit. The sounder would then 

 be a relay. The circuit which is closed by the 

 relay may be hundreds of miles long or it may 

 be only a few feet. A relay used in this man- 

 ner to send a message on to a more distant 

 station is called a repeater. At the farther end 

 of the second line there may be a second relay 

 which closes a third circuit. A message may be 

 sent to any desired distance by using a suffi- 

 cient number of relays. 



The Codes. There are two codes in general 

 use, the Morse and the Continental. The Morse 

 code is used in the United States and Canada, 

 and the Continental code has been adopted in 

 all other countries. For communication by 

 cable between the United States and other na- 

 tions the Continental code is used. The sig- 

 nals of the Continental code consist of dots and 

 dashes ; those of the Morse code consist of dots, 

 dashes and spaces. The letters which occur 

 most frequently have the simplest symbols ; for 

 example, the letter e is represented by one dot 

 and the letter t by one dash in both systems. 

 A dot is made by quickly pressing and releas- 

 ing the key, producing a rapid click-clack sound 

 in the receiver. A short dash is twice as long 



F 



H 



/ / X If . N 



OP R S T 



> 



V \S X Y Z. 



mm 



.... 



NUMERALS 

 23 

 wwwwmw 



6 7 



4 

 



8 



PMCWNIOH ? 



Period Semi-Colon Interrogation 



THE MORSE CODE 



Dots, dashes and spaces are employed to form 

 the letters. 



as a dot. A long dash, as for the letter I, is 

 equal to four dots. The space between the 

 dots and dashes that make up a letter is equal 

 in length to the dot. The space between the 

 letters of a word is equal to three dots and the 

 space between words to six dots. A space which 



