Cathode Ray Tube
One of the most exciting uses of electron beam is the cathode ray tube (CRT). This may not mean much, until you learn that for many years, the television screen is one end of a cathode ray tube. Although many televisions today use LCD instead of cathode ray tube, cathode ray tube is still in use not only in TV but in many other devices.
A cathode ray tube is usually a large glass tube. At one end is the source of electron beam that usually consists of a hot filament and one or more positively charged metal rings. This setup is called an electron gun and produces a beam of electrons.
The other end of the the glass tube is usually enlarged into a flat area. The inner wall of this area is coated with layer of fluorescent chemical. This coating gives out light when electrons fall on it. When the electron beam is focussed onto a very small area on the fluorescent screen, we see a small, bright dot.
The traditional television works by moving this dot rapidly to draw pictures repeatedly on the screen. Because electrons are so light, it is possible to draw hundreds of new pictures every second, so that we can watch a video on the screen. There are two ways of move the electron beam. We can either use a magnetic field or an electric field.
The traditional television uses an magnetic field to move the beam. This field comes from coils of wires around the cathode ray tube. When there is a current in the coil, the coil produces a magnetic field that goes through the glass wall of the tube and reaches the beam. The beam is really an electric current. So when the beam flows through the magnetic field, it experiences a force according to Fleming's left hand rule, and gets deflected to one side. So by placing a few coils around the tube and applying suitable currents, the electron beam can be made to draw any picture on the screen.
Another way to move the dot is to use an electrical field. A simple way is to place metal plates on two sides of the tube and connect these plates to a voltage. One plate becomes positive and the other becomes negative. As a result, the beam will get attracted to one side. So by placing a few plates around the tube and applying suitable voltages, we can also draw any picture we like.
