RADIO 



283 



angle file. Study the construction carefully. If no tube 

 is available study Figure 463. Let these questions guide your 

 observations. How many connections are made through the 

 base of the tube? Can you locate a fine wire running from 

 the glass base up the center of the tube and back? This is the 

 filament. The coil of fine wire surrounding the filament is 

 the grid. The solid metal plate around the grid is the 

 plate. Do any of these touch each other? When your file 

 first punctured the tube what did you observe ? Why is this 

 called a vacuum tube ? 



NOTE: Some reference reading on the construction and 

 operation of both radio broadcasting and receiving sets 

 should be done before performing the next experiment. 



Experiment 171. What principle is used to tune a re- 

 ceiving set to the wave of a broadcasting station? 



Stretch a wire between two supports as shown in Figure 

 464. From the wire suspend two pendulums, each having 



a length of about three 

 feet, and two others about 

 twenty inches in length. 

 Arrange them alternately 

 as shown in the diagram. 

 One of the long pendulums 

 represents a broadcasting 

 station sending out a cer- 

 tain long wave length. 

 The other long pendulum 

 FIG. 464 represents a receiving set 



timed to this wave length. 



In the same manner one of the short pendulums represents 

 another broadcasting station tuned to a shorter wave length 

 and the other short one, a receiving set tuned to receive 

 the shorter wave length. 



Start one of the long pendulums swinging and observe 

 the result. Stop the long pendulum swinging and start one 

 of the shorter ones swinging. Repeat this several times and 

 see if you can discover how it illustrates the principle of 

 tuning a radio set to the wave length of a certain broad- 

 casting station. 



In your notebook record your notes and complete the fol- 

 lowing statements. 



When the long pendulum is started swinging its motion 



is carried by the to the pendulum which has the same 



and starts it The shorter ones are not by 



this. When the long pendulums are stopped and one of 

 the short pendulums is started swinging, the other short 



pendulum , but the long ones this time are This 



experiment shows that at any one time a receiving set may 



be tuned to only broadcasting wave length. In this 



experiment the longer receiving pendulum could have been 

 tuned to the shorter broadcastng wave by it. 



OTHER INVESTIGATIONS WHICH YOU CAN MAKE 



1. Visit a broadcasting station. 



2. Build a simple crystal radio receiving set. 



3. Build a simple tube radio receiving set. (See Caldwell, 

 Eikenberry, and Glenn, Laboratory Experiments in Gen- 

 eral Science.) 



READINGS WHICH WILL HELP ANSWER THE 

 PROBLEM QUESTIONS 



How are radio and wireless alike? In the recent 

 study of wireless you learned how messages might be 



FILAMENT 



sent in code without wires. This was made possible 

 by setting up electrical disturbances with surging 

 electrons and then, with a coherer or a crystal, de- 

 tecting the waves. 



Radio is different from wireless in the same way 

 that the telephone is different from the telegraph. In 

 the telegraph, messages are sent in code, while in the 

 telephone the conversation and sounds may be 

 changed into electric currents and carried over the 

 wires. 



In radio, sounds are changed into electrical waves 

 and broadcast without wires. While it is not practi- 

 cable to use a spark coil 

 to send out radio waves, 

 the simple crystal re- 

 ceiving set used to pick 

 up the wireless waves 

 may also be used to 

 pick up radio programs. 

 This is the experience 

 of many who have built 

 crystal receivers. Fig- 

 ure 459 shows a draw- 

 ing of a simple crys- 

 tal receiving set which 

 may be used to hear ra- 

 dio broadcasts. 



The development of 

 the vacuum tube made 

 radio possible. A few 

 years after the inven- 

 tion of the electric lamp 

 Edison discovered that 



PLATE 



'A" BATTERY "E.' BATTERY 



FIG. 465. THE FLEMING VALVE 



electricity could be made to flow in only one direction 

 through the space around the hot filament of his bulb. 

 This is known as the Edison effect. It was put to 

 work by Fleming, an English scientist, who added a 

 "plate to the inside of the electric bulb as shown 

 in Figure 465. This is known as the Fleming valve. 

 When the filament was heated with current from an 

 "A" battery and a "B" battery connected in such a 

 way that the plate was always kept positive (see Fig. 

 465), the valve could be used to detect wireless wave- 

 This proved to be much more sensitive than either 

 the coherer or crystal for receiving wireless waves. 



A few years later Dr. Lee DeForest further im- 

 proved the Fleming valve by placing a third element, 

 called the grid, between the filament and plate as 

 shown in Figure 466. This is the modern vacuum tube 

 used so extensively in communication. 



Not only have vacuum tubes improved the recep- 

 tion of wireless and radio waves, but they have made 

 radio broadcasting possible. Figure 458 shows a mod- 

 ern vacuum tube used in broadcasting stations. 



What is a vacuum tube and how does it work? 



