RADIO 



285 



trolled by the sending key. A long series is a dash and 

 a short series is a dot. When electrons are made to 

 surge in a vacuum tube a high frequency radio wave 

 is sent out. This is an undamped or continuous wave 

 which cannot be heard by ear and will not operate 



the telephone receivers. 

 Figure 469a shows an 

 undamped or continu- 

 ous wave of very high 

 frequency sent out by a 

 vacuum tube. 



The undamped or 

 carrier wave, as it is 

 sometimes called, must 

 be changed before start- 

 FIG. 469. UNDAMPED WAVE AND in g ts journey from the 

 MODULATED WAVE broadcasting antenna. 



It must have the voice 



wave placed upon it. This is accomplished by plac- 

 ing a sensitive telephone transmitter, called a micro- 

 phone, in the plate circuit and speaking, singing, or 

 playing into it. From your study of the telephone, 

 recall that as sound waves affect the transmitter they 

 are changed into electric currents which pass through 

 the wire. Voice waves are sent into the plate circuit 

 of a vacuum tube at the broadcasting station. At the 

 same time electrons are surging in the tube, producing 

 a carrier wave as shown at a. The voice wave is placed 

 upon the carrier wave in such a way as to produce 

 a combination or modulated wave as shown at b. This 

 modulated wave is sent out from the antenna of the 

 broadcasting station. The voice wave of the micro- 

 phone may be produced by any sort of sound dis- 

 turbance in the studio and will modulate the carrier 

 wave in such a way that the same sound will be repro- 

 duced in the loud speakers of receiving sets. 



Exercise. Write a summary statement showing clearly 

 each step involved in changing a sound wave into a modu- 

 lated ether wave that is sent out by a broadcasting station. 

 How does a radio receiving set work? From your 

 study of the crystal wireless receiver, recall that the 

 crystal was used to change the alternating-current 

 ether wave into a direct-current wave which would 

 operate the telephone receivers. When the vacuum 

 tube is used to detect either wireless or radio waves 

 its action is quite similar, that is, it changes the al- 

 ternating-current, high frequency waves into a direct- 

 current wave which will operate the ear phones or 

 loud speaker. Let us study how this is accomplished 

 by the tube. 



Figure 466 shows a vacuum tube connected into a 

 circuit. Notice that the antenna is connected to the 

 grid ; that the "A" battery supplies current to light the 

 filament of the tube, and that the "B" battery has its 



positive pole connected to the plate. This keeps the 

 plate always positive. 



As current flows through the filament of the tube 

 electrons are thrown out into the space around it. This 

 is what caused the effect noticed by and named for 

 Edison. You will recall from previous experiments 

 that electrons are negative particles of electricity. The 

 plate is always positively charged by being connected 

 with the positive side of the "B" battery. Being posi- 

 tively charged means that there is a shortage of elec- 

 trons on the plate, and therefore many of the electrons 

 thrown out by the hot filament will rush across to 

 the plate and then on to the "B" battery. A stream 

 of electrons is an electric current, and if a delicate am- 

 meter is placed in the plate circuit it will show that 

 a current is flowing. 



By reference to Figure 466 you will see that the 

 grid is connected with the antenna. Let us see what 

 effect the grid will have upon the flow of electrons 

 from the filament to the plate. Do not forget that the 

 grid is located directly between the filament and the 

 plate. 



As the ether waves from the broadcasting station 

 cut across the antenna wires, weak alternating cur- 

 rents of very high frequency are set up and carried to 

 the grid. From your study of electricity you know that 

 an alternating current flows first in one direction and 

 then in the other. First it is positive and then negative. 

 As such a current flows through the grid it charges the 

 grid positively for a brief fraction of a second, then 

 negatively, and then repeats. When the grid is posi- 

 tive it will help the plate as electrons from the filament 

 are attracted to it. When the grid is negative it will 

 repel the electrons from the filament, and no current 

 will flow. In the next instant the grid charge will 

 change to positive and electrons will flow to the plate 



Aerial 



"A" battery 



Aer 



"B" battery 



"A" battery 



FIG. 470. VACUUM TUBE SHOWING FIG. 471. VACUUM TUBE SHOW1NV, 



GRID POSITIVE AND ELECTRON GRID NEGATIVE AND NO ELECTRON 

 STREAM FLOWING THROUGH STREAM FLOWING 



PLATE 



