Thermionic emission and the cathode-ray tube (CRT)

• When a metal cathode is heated by a filament, some electrons gain enough thermal energy to leave the surface: this is thermionic emission.
• This is the working principle of the cathode-ray tube found in old televisions and monitors.

Main parts of a CRT:

1. Electron gun (E-gun): a heated cathode emits electrons; a grid (negative voltage) controls how many electrons pass, so it controls the brightness; the anodes (positive) accelerate the electrons and focus them into a thin beam.
2. Deflecting plates (X and Y) (or magnetic coils): electric or magnetic fields bend the beam so that it scans the screen point by point in lines (a "raster").
3. Fluorescent screen: when the electrons strike it, it emits light. The brightness of each spot follows the signal applied to the grid, so the image is built up point by point. The screen is connected to the anode, so the electrons that arrive complete a circuit and give a current.