250 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1938 



.GRID 

 x -PLATE 



CONTROL OF ELECTRON TUBES 



DeForest added a third electrode, known as the grid, to the electron 

 tube. In application in the tube, the grid (fig. 5) is a screen or wire 

 mesh placed between the emitter or cathode and the anode or plate. 

 The grid functions by means of a change of potential or charge upon 

 its surface. This change of potential on the grid becomes an addi- 

 tional force acting upon the electrons surrounding the cathode and 

 because of its nearness to the cathode exerts a very powerful influence 

 upon the electron stream to the anode. The figure is a cross-sectional 

 view which shows the usual location of the control grid in a three- 

 electrode tube or triode. If a small positive potential be placed upon 

 the grid, it will exert a relatively powerful pull upon all electrons 

 which emerge from the surface of the cathode. This pull will be 

 added to the effect of their initial velocity of emission and will oppose 

 the repelling effect of space charge. The result of this action is that 



many electrons will move out farther from 

 the cathode and come under the influence 

 of the positive anode and thus pass to 

 the anode. Any increase in the positive 

 potential of the grid will cause still more 

 of the electrons to move to the plate. 

 Conversely, a small negative potential on 

 the grid will repel every electron that 

 emerges from the cathode and add its 

 weight to the effects of space charge and 

 cathode attraction. Thus many electrons 

 which otherwise would be carried far 

 enough by their initial velocity of emission 

 to travel to the plate will now be held in 

 check by the repelling force of the negative grid. It is obvious that 

 a sufficiently negative potential on the grid would bar all electrons 

 from ever reaching the anode. Thus it is apparent that a suitable 

 variation of potential placed on the grid will control accurately the 

 flow of current in the cathode-anode circuit. Since a mere change of 

 potential does not involve any expenditure of power we have the 

 strange phenomenon of controlling a large amount of power with- 

 out the use of any power. In the actual application of the triode 

 or three-electrode tube, the circuit for the grid does use some small 

 amount of power. The triode thus is inherently an amplifier of 

 alternating currents and as such it finds its greatest application. 



Figure 5.- 



-Diagram of three-eloctrode 

 tube or triode. 



USE OF ELECTRON TUBES 



Ra dio signals of telegraph, voice, or television are sent out through 

 space by means of very high-frequency alternating currents called 

 carriers. This method is necessary since lower frequencies will not 



