286 



COMMUNICATION 



again. Thus the grid is a valve which allows electrons 

 to flow from filament to plate one instant and prevents 

 them from flowing the next. That is, it changes the 

 alternating currents in the grid circuit to direct cur- 

 rents in the plate circuit. Ear phones or a loud speaker 

 are in the plate circuit where they are operated by the 

 direct current resulting from the rectifying action of 

 the vacuum tube. The currents set up in the antenna 

 by the broadcast waves are very weak and could hard- 

 ly be used to operate ear phones. Before it is possible 

 for these small currents to operate a loud speaker or 

 ear phones, they must be amplified. This is accom- 

 plished in the receiving set either by a series of vac- 

 uum tubes or by transformers. In most sets both 

 tubes and transformers are used. 



Radio receiving sets may be tuned to different fre- 

 quencies. Many of you have tuned radio receiving 

 sets to different stations and have wondered how this 

 is accomplished. Recall from Experiment 171 that a 

 swinging pendulum could start another of the same 



FIG. 472. RADIO RECEIVING SET 



length swinging but would not affect pendulums of 

 different lengths hung from the same wire. The vibra- 

 tions set up by the swinging pendulum were carried 

 along the wire to the other one of the same length. 

 The two pendulums were tuned to the same fre- 

 quency. One represented a broadcasting station and 

 the other of the same length a receiving set. The pen- 

 dulums which were longer or shorter could have been 

 tuned to the frequency of the swinging pendulum by 

 making them the same length. 



Tuning a radio set is accomplished through a little 

 different method. Each broadcasting station has a set 

 frequency, that is, a definite number of vibrations per 



second for its carrier wave. This frequency is set by 

 the Federal Radio Commission and must be strictly 

 adhered to. Suppose a station is broadcasting on a 

 frequency of 550 kilocycles. "Kilo" means thousand. 

 Therefore the carrier wave of this station would be 

 making 550,000 vibrations per second. If you wish to 

 get the program of this station on a receiving set, you 



Sound Waves to 

 Electrical Wanes 



Electrical Ci/aues 

 amplified 



(Vaues cut info 

 disk. 



Disk to 

 Electrical Waves 



Electrical Waves 

 amplified 



Loud 



FIG. 473. SCHEMATIC OF RECORDED SOUND WAVE ON RECORD 



must "tune" the set until it will respond to this fre- 

 quency just as one pendulum responded to the vibra- 

 tion sent along the wire by the one of the same length 

 while vibrating. Tuning is accomplished by turning 

 the dials of the set. This turns coils or variable con- 

 densers or both in the circuit until it is in tune with 

 the broadcast wave desired. 



What are some of the recent advances in the appli- 

 cation of the principles of radio? Not only has the 

 radio added greatly to home entertainment, but it has 

 become very important as an agency of communica- 

 tion. Trans-oceanic telephony is made possible by 

 radio. Communication from airplanes and the radio 

 compass could hardly be dispensed with. Short wave 

 broadcasting is used by many city police departments 

 to communicate with roving police cars which are 

 thus enabled more quickly to apprehend criminals. 

 Radio made possible the daily communication with 

 Admiral Byrd when he was thousands of miles away 

 in the frozen Antarctic continent. 



The past few years have seen not only the remark- 

 able developments of the radio but also sound motion 

 pictures, sending pictures by wire, and television or 

 sending scenes. It is possible that television may soon 

 be perfected to the point where radio sets will be 

 equipped to receive scenes sent without wires. 



There are two methods of recording sound for 

 motion pictures, one in which the sound waves are 

 placed upon a record as for the phonograph and the 

 other in which the sound wave is placed on the same 

 film as the picture. Sound placed upon a record is re- 

 produced in much the same manner as a phonograph 



