284 



COMMUNICATION 



From Experiment 170 you have learned that the mod- 

 ern vacuum tube has three essential elements. These 

 are the filament, the grid, and the plate. Figure 463 

 shows a vacuum tube with the glass bulb removed in 

 one case, and the plate removed in another. The four 

 prongs at the bottom are the connections. Two of 

 them connect with the filament, one with the grid, 

 and one with the plate. 



-FRAMENT 



i-GRID 



rPLATE 





&ROUND 



FIG. 466. DE FOREST'S IMPROVEMENT OF THE FLEMING VALVE 



Among the various uses of vacuum tubes at the 

 broadcasting station we shall consider only two. The 

 tube may be used to set up electrical disturbances, 

 or oscillations, just as the spark coil was used in the 

 simple wireless set. It is better than the spark coil 

 for this purpose because the surging electrons may 

 be more carefully controlled in the tube. Another use 



to which the vacuum tube is put in both broadcasting 

 and receiving sets is to build up or amplify electrical 

 waves which are weak. Thus by use of amplifying 

 tubes the normal voice of a speaker may be made 

 thousands of times stronger. Tubes are used for this 

 purpose in modern public address systems so com- 

 monly used in large auditoriums and in outdoor am- 

 phitheatres. 



What happens at the broadcasting station? Figure 

 467 shows an interior view of a broadcasting studio. 

 This picture gives little evidence of the complex elec- 

 trical circuits, tubes, and generators which are "be- 

 hind the scenes" but which are necessary before the 

 sound made by the instruments or voices of the per- 

 formers can be changed to ether waves and sent out. 

 Figure 468 shows some of the equipment in a mod- 

 ern broadcasting station. 



Recall from your study of wireless that ether waves 

 can be set up by the surging electrons of an electric 

 spark. Similar ether waves can be set up by electrons 

 surging in a vacuum tube. Tubes have largely re- 

 placed spark coils for wireless sending. 



Courtesy Radio Station KMOX 



FIG. 467. INTERIOR VIEW OF BROADCASTING STATION 



Courtesy Radio Station KMOX 



FIG. 468. EQUIPMENT USED IN BROADCASTING STATION 



Ether waves such as those which are set up by the 

 surging electrons in a spark coil are vibrating many 

 thousand times a second and are too rapid to be heard 

 by the human ear. We speak of them as radio- frequency 

 waves. Each spark sends out a little train of waves which 

 soon die out. These are spoken of a damped waves, and 

 each series will give an impulse to the telephone re- 

 ceiver. The length of these little wave series is con- 



