ELECTRICAL COMMUNICATION 



165 



81. Electrical Communication. Experiment 87. Attach one 

 end of a wire to a pole of a dry cell and the other end to one of the 

 binding posts of a telegraphic sounder. From the other binding post 

 of the sounder lead a wire to the binding post of a telegraphic key. 

 Connect the free binding post of the key with 

 the free pole of the, battery (Fig. 76). When 

 the key is pushed down, the circuit is closed and 

 the sounder clicks. If a relay can be procured, 

 remove the sounder and connect two of the bind- 



SOUNDER 



LINE BATTERY 



Fig. 76. 



Fig. 77 



ing posts of the relay in the same way that the sounder was connected. 

 Connect one of the free binding posts of the relay with a binding 

 post of the sounder and the other binding post with the pole of a 

 dry cell. Connect the other 

 pole of the dry cell with the 

 free binding post of the 

 sounder. When the key 

 closes the circuit through j 

 the relay, the circuit through 

 the sounder and its dry cell 

 is closed by the relay (Fig. 

 77) and the sounder clicks. 



Fig. 78. 



This is the usual arrange- 

 ment in a simple telegraph 

 office. The sounder in the first part of the above experiment can be 

 replaced by an electric bell (Fig. 78) and the key by a push button, 

 thus showing the arrangement of the ordinary doorbell. 



Electricity can be developed by chemical action as well 

 as by friction, and many different kinds of electrical cells 

 have been invented. The most simple of these is a sheet 



