THE EARTH AND THE TELEGRAPH 223 



The telegraph is now such a universal means of communi- 

 cation between distant points that one wonders how business 

 was conducted before its invention in 1832 by S. F. B. Morse. 

 Shortly after the invention of telegraphy, operators learned 

 that they could read the message by the click of the marker 

 against a metal rod which took the place of the tape. In all 

 telegraph offices of the present day the old-fashioned tape is re- 

 placed by the sounder, shown in Figure 93 . When current flows, 

 a lever, Z,, is drawn down by the electromagnet and strikes 



against a solid metal piece 

 with a click ; when the cur- 

 rent is broken, the lever 

 springs upward, strikes an- 

 other metal piece and makes 

 a different click. It is clear 

 that the working of the key 

 which starts and stops the 

 current in this line will be 



imitated by the clicks of the sounder. By means of these 

 varying clicks of the sounder, the operator interprets the 

 message. 



The earth an important part of a telegraphic system. 

 In the early telegraph lines, two wires were used, as in 

 Figure 91 ; then it was found that a railroad track could be sub- 

 stituted for one wire, and later that the earth itself served 

 equally well for a return wire. In the present arrangement 

 there is but one wire, the circuit being completed by the earth. 

 No fact in electricity seems more marvelous than that the thou- 

 sands of messages flashing along the wires overhead are likewise 

 traveling through the ground beneath. If it were not for this 

 use of the earth as an unfailing conductor, the network of over- 

 head wires in our city streets would be even more complex than 

 it now is. 



