What is the use of a control grid in cathode ray tube?

Answer 1

The grid in a cathode ray tube is used to control the beam current. The grid in the CRT is positioned near the cathode, and between it (the cathode) and any other elements, like those for focusing. The cathode is the (cylindrical) element that is coated with a metal that has good thermionic properties. That means that as it gets hot, electrons from within its structure reach such high thermal energies that they can actually leave and hang around outside of the metal itself. (Recall that the tube is very highly evacuated.) There is a heater inside the cathode to heat it up to set up the thermionic emission and the space charge. The grid is actually close to the cathode so it can affect this space charge. If the grid is driven negative, the negative grid will "push" on the space charge (the cloud of negative electrons) and keep it in place. (The tube is said to be cut off.) As the grid is driven positive, it will start attracting electrons. Yes, some will actually go to the grid, but the majority will, when flying away from the cathode (and becoming cathode rays) be "caught" by the high voltage on the anode. The stream of electrons, called a beam (because it's focused) will be pulled past the grid to go do their thing at the anode itself. The anode is actually the phosphor coating on the inside of the tube at the "front" where the viewer looks. Electrons smack the coating on the inside of the glass of the tube at a point where they are aimed and ionize it causing it to emit photons. The photons (light) will travel through the glass and out of the tube to the viewer's eyes. 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.