ELECTRON THEORY KLOEFFLER 



253 



potential. This action is due to the fact that electrons emitted by 

 the cathode land on the grid and make the grid negative. This 

 negative grid repels nearly all electrons and does not permit many 

 to pass through the meshes of the grid to the plate. If the grid is kept 

 negative enough, the electrons cannot attain sufficient velocity to 

 cause ionization and the tube is nonconducting between cathode and 

 anode. As the grid is made less negative a point is reached where 

 the electron velocity is great enough to cause ionization. Positive 

 ions will then form a sheath around the grid and neutralize its effect 

 and a large current flows to the anode. This current is limited only 

 by the impedance of the anode circuit. Once started, the cathode- 

 anode current cannot be stopped by any change of potential on the 

 grid. If the grid circuit is opened, the positive ions will neutralize 

 any electrons which land on the grid, and if the grid be made more 

 negative, it will attract positive ions to its meshes and the resulting 

 positive ion sheath will again neutralize the negative grid so that the 

 cathode-anode current stream will be 

 unaffected. The only way the anode 

 current can be stopped when direct cur- 

 rent is applied to the plate is to open 

 the anode circuit. 



The hot-cathode grid-controlled gas- 

 eous-conductor tube or thyratron finds 

 its usefulness when an alternating poten- 

 tial is applied to its anode circuit. Again, 

 the tube can be started by placing a 

 positive potential on the grid and the tube 

 will function as a half-wave rectifier as 

 long as the grid remains positive. If the 

 positive potential be removed from the 

 grid, the anode current will go to zero at the next negative wave of 

 potential. Thus the starting and stopping of the rectification process 

 can be controlled simply and perfectly (without arcing) by the appli- 

 cation of a continuous potential to the grid. The grid may also be 

 excited by a low alternating potential from the same source as the 

 anode supply. If the grid and anode potential are in step, complete 

 single-wave rectification will occur. If the grid potential be made to 

 lag the anode potential, then rectification will start after the beginning 

 of the cycle and the time and magnitude of the rectified current can 

 be controlled. Thus a shift of phase of the grid voltage through 180° 

 will vary the rectified current from zero to a maximum of half-wave 

 rectification. The thyratron gives fine control of rectified current 

 and finds a wide application in theater-stage lighting, flood-lighting, 

 motor speed control, and elsewhere. 



Figure 9. — Diagram of a grid-controlled 

 gas-filled rectifier. 



