The electron gun
The electron gun produces a fine beam of electrons. It consists of a glass tube at a very low pressure. Electrons are produced by thermionic emission from a tungsten filament heated by a 6 V supply. A high positive potential is connected to a cylindrical anode (+). The electrons will accelerate to a high speed and shoot straight through the hole of the anode in a fine beam of electrons. The electron beams are sometimes called cathode rays because they are emitted from the cathode.
Deflection by electric field
When a potential difference is applied across to parallel plates placed horizontally and some distance apart, an electric field is produced. When an electric beam is passed between these plates, the beam will deflect towards the positive plate due to the attraction force between opposite forces. The greater the electric field strength, the larger the degree of deflection of the electric beam.
Deflection by magnetic field
A magnetic field is applied at right angles to the beam of electrons by placing two poles of a magnet. The electron beam is deflected by its’ magnetic field. Electrons are negative and therefore a beam of electrons would be equal to a positive or conventional current in the opposite direction, and by applying Fleming’s Left-hand Rule, the deflection would be downwards. This experiment provides a strong evidence that cathode rays are negatively charged electrons.