THE TELEPHONE 345 



used to send out waves through the ether in every direction. 

 These waves are picked up by a sensitive receiving apparatus, 

 which sets in motion another diaphragm with vibrations 

 corresponding to the sending diaphragm, and thus the 

 original sound, with all its tones and quality, is reproduced. 



Types of transmitters. A variety of transmitters have 

 been devised for controlling the current. One is very much 

 like the ordinary carbon microphone transmitter. In an- 

 other form, called an audion, a flame is used as a part of 

 the circuit, as it is found to be very sensitive to electric 

 waves. Still another depends on the fact that the elec- 

 trical resistance of selenium depends on the amount of light 

 that strikes it. This is used in connection with an arc 

 lamp. 



The sending station has aerial wires like a telegraph station 

 and may send out continuous electric waves ; or it may send 

 out a discontinuous set of waves caused by sparks, which 

 vibrate so rapidly that they cannot be heard, and do not 

 affect the telephone diaphragm. This vibration frequently 

 may vary from 40,000 to 100,000 per second. 



Tuning. The wireless telephone has the same problem 

 of tuning as the wireless telegraph. This difficult problem 

 has been partially solved, so that a receiving instrument 

 can receive a message only when it is tuned to correspond 

 to the sending instrument. 



SUPPLEMENTARY QUESTIONS FOR CLASS DISCUSSION 



1. Which is more useful to man, the telegraph or the tele- 

 phone ? 



2. What has been the history of the development of the 

 telephone ? 



3. How are the receiver and transmitter constructed? 



4. How are the connections made at central? 



5. How does the wireless telephone differ from the ordinary 

 telephone ? 



