COMMUNICATION BY TELEGRAPH 



267 



The switch on the key allows to flow around the 



contact points, closing the through the key. 



Experiment 163. How are messages sent over long 

 distances by telegraph? 



For this experiment you will need two keys, two sound- 

 ers, and three or four dry cells. String a wire from one 



FIG. 427 



end of your room to the other and connect the instruments 

 and batteries as shown in Figure 428. Instead of a return 

 wire connect the instruments at either end to a radiator 

 or water pipe. 



FIG. 428 



Close the key-switch at the end of the line where the 

 first message is to be received and leave the switch open 

 at the sending end. Have your partner send to you and then 

 by changing the switch at each end you may send to him. 

 Study the set-up carefully and trace the current through 

 all the instruments. 



Write a paragraph carefully summarizing the important 

 conclusions to be drawn from this experiment. 



OTHER INVESTIGATIONS WHICH YOU CAN MAKE 



1. If a telegraph relay is available, connect it in one 

 end of the telegraph line used in Experiment 163. 



2. Visit a telegraph office and examine the equipment 

 used. 



3. Devise an experiment to prove that the earth is a con- 

 ductor of the electric current. Test your experiment. 



READINGS WHICH WILL HELP ANSWER THE 

 PROBLEM QUESTIONS 



What is the early history of the telegraph? As 



early as 1684 Robert Hooke, an English scientist, 



had devised a scheme for aerial communication. 

 In his scheme, towers were built on hills several miles 

 apart and messages were sent by pulling the letters 

 of the alphabet to the tops of these towers. The tele- 

 scope was used in reading the letters, and in this man- 

 ner messages could be sent from station to station. 

 Over a. hundred years later, during the French Revolu- 

 tion, Chappe, a Frenchman, worked out a plan of 

 semaphore signaling. This made use of arms placed 

 on a high tower. These arms could be set at different 

 angles by means of cords. Chappe worked out a code 

 in which different positions of the arms represented 

 different letters of the alphabet. By 1800 this system 

 was used extensively over France for purposes of 

 communication between cities. Messages could be 

 sent at about 100 words per hour. 



In 1748 Benjamin Franklin had been able to send 

 an electric discharge through a wire across the Schuyl- 

 kill River at Philadelphia and had thought of the pos- 



Courtesy Western Union 



FIG. 429. MORSE'S TELEGRAPH INSTRUMENTS 



sibilities of using electricity for purposes of com- 

 munication, but too little was then known about this 

 mysterious force. By 1800 Galvani and Volta, Italian 

 scientists, had made their discoveries which made pos- 

 sible the electric battery. The next quarter century 

 saw the discovery of the magnetic effect of the electric 

 current by Oersted ; the series of discoveries by Stur- 

 geon in 1825, Faraday about 1830, and Henry about 

 1831 perfected the electromagnet. All of these dis- 

 coveries were necessary to pave the way for the in- 

 vention of the telegraph. 



