Electron beams: This is how tube TVs, shadow cross tubes, etc. work

Even if old tube televisions are no longer common, their operation using electron beams is particularly interesting. In this article we reveal what an electron beam is, how it spreads in space and how it is used in televisions.

Electron beams in shadow cross tubes

An electron beam is a bundle of electrons that is released by a hot cathode and accelerated by an electric field. But how does it work in detail?

• The physicist Sir William Crookes invented the shadow cross tube to find out how electron beams behave. It consists of an evacuated glass tube with a hot cathode on one side, which is energized. A little further away from the hot cathode is an annular anode. There is a shadow cross further back in the cylinder. In the end, a fluorescent layer is applied to the glass.
• There is a filament in the hot cathode, to which a low alternating voltage is applied. This causes the filament to glow, causing electrons to detach and form an "electron cloud". There is a negatively charged Wehnelt cylinder around the filament that focuses the electron beam. The ring-shaped anode is charged positively. This attracts and accelerates the electrons from the hot cathode. They shoot through the ring of the anode as an electron beam.
• The shadow cross is grounded: all the electrons hitting it are removed. All electron beams that are not intercepted by the shadow cross make the fluorescent layer glow.
• It is striking that the shadow of the cross, which is created by the light of the incandescent filament, looks exactly like the shadow of the cross, which is caused by the electron beams on the fluorescent layer. From this it can be deduced that electrons spread in the field-free space as straightly as light and are not deflected when flying past an uncharged metal cross.
• In another experiment, the glass jar was filled with neon so that the electron beam was visible and two capacitor plates were attached inside. It was striking here that the deflection of the electron beam in the direction of the positive plate increased as the plate voltage increased, but decreased again as the acceleration voltage increased.
• Conclusion of the experiments: Electron beams can be influenced by both magnetic and electrical fields.

This is how a tube TV works

A tube screen is similar to a shadow cross tube. There is also an electron gun at the end, consisting of a cathode (in the Wehnelt cylinder) and anode.

• However, further ahead in the television (outside the tube) there is a pair of coils arranged vertically and horizontally. The vertically arranged pair of coils determines the horizontal deflection and vice versa. The advantage of the coils is that - unlike capacitor plates - the electron beam can also be guided precisely in the edge area.
• There is a special fluorescent layer on the front of the television that lights up as soon as it is bombarded with electrons. That was the setup for a classic black and white television.
• With color TVs, there is not just one electron gun, but three. There is also a shadow mask just in front of the screen. There is a separate electron gun for each color (red, green, blue). The screen consists of many red, green and blue dots so that each electron gun can target its own color pixels. The shadow mask ensures that, for example, the electron gun for the blue pixels does not accidentally light up a red pixel.
• Sawtooth voltages are applied to the coils so that the beam is slowly pushed from left to right and then quickly jumps back to the starting point. He also moves down a bit per row and jumps back to the beginning at the end. To prevent flickering, the colors are controlled at different times so that, for example, only the upper corner is not illuminated at times.

In the next practical tip, we will give you an introduction to quantum physics.