What is the use of a control grid in cathode ray tube?
Recall that the grid is close to the cathode, and it tells the space charge what to do. If the grid is negative, the space charge huddles near the cathode and the tube is cut off. If the grid becomes a bit positive, some electrons zip out from the cathode, past the grid and on out to the anode. The more positive the grid, the more electrons are called out from the cathode to make the trip to the anode. Moving electrons are current. And the grid is controlling the amount of beam current.
