USES OF ELECTRICAL ENERGY 169 



193. Wireless Telegraphy. Messages are now trans- 

 mitted without wires over long distances by means of 

 waves in the ether. These waves are caused when elec- 

 trical discharges are produced. They travel with great 

 speed in all directions from their source, growing 

 weaker as they advance. Of course these waves can do 

 no great work at a distance ; but delicate instruments 

 are made, by means of which the weak waves serve to 

 close the circuit of a local battery and telegraph receiver, 

 so that this receiver shall be operated by them. 



QUESTIONS 



1. Explain the action of the electric motor. What sort of cur- 

 rent can be used ? Name uses of electric motors. 



2. How are electric cars driven ? How is the speed of the 

 motor and car controlled ? How is the main circuit completed ? 



3. Explain the telephone. What travels over the wires? How 

 are the sound waves that the listener receives produced ? 



4. How is a telegraph message sent by an operator? Describe 

 the sounder on which it is received. Explain the action of the 

 sounder. 



5. Carefully explain the common electric bell. 



6. What would you do with an article that you were going to 

 plate ? What is put into the water ? What is the action of the 

 current upon this substance ? 



7. Of what electrical effect do electric lights make use? 

 Explain how a current is used to make a body luminous. What 

 two sorts of electric lamps are common ? 



8. Explain the incandescent light. Why is the carbon thread 

 placed in a vacuum ? 



9. Explain the arc light. What sort of a current is needed, 

 and why ? From what are the light waves sent out ; that is, 

 what parts of the lamp are luminous? 



