Question

What is the function of the 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.