How is the image obtained on the TV picture tube screen?
The image that we see on the screen of an old TV is reproduced on the screen of a cathode ray tube – a kinescope. The kinescope is a glass flask with a narrow cylindrical part – a neck that passes into a conical part and ends with a wide bottom.
On the inside, the bottom of the picture tube is covered with a special compound – a phosphor, which starts to glow when bombarded with electrons. The bottom of the tube is coated with a layer of phosphor and forms a picture tube screen. The air from the flask is evacuated, since electrons can only move freely in a vacuum.
An electron “gun” is placed in the throat of the picture tube – a device that creates a narrow directed stream of electrons – an electron beam. The electron gun has a cathode, an anode, and several electrodes for focusing the beam control.
The cathode is heated by the filament and emits electrons. It is surrounded by a metal cylinder with a small hole in the middle through which the electrons it emits pass; this is the control electrode of the CRT. Behind it are the accelerating and focusing electrodes and, finally, the anode. All these electrodes and the anode are hollow cylinders and differ from each other only in length and diameter.
A positive voltage is applied to the accelerating electrode. The electrons flying through it get acceleration and move on. The focusing electrode collects the flow of electrons into a narrow beam. A very high positive voltage is applied to the anode, which is connected to a conductive coating applied to the inner surface of the tube’s cone. The electrons under the action of this voltage acquire an even greater speed of movement towards the screen: they fly out of the electron “gun”, like projectiles from a gun barrel.
The stronger the electron flow, the brighter the screen glow. The “density” of the electron beam and, therefore, the brightness of the image changes under the action of the control electrode of the kinescope, which plays the same role as the grid of the triode.
The movement of the beam across the CRT screen occurs with the help of a magnetic field, which is created not by bulky magnets, but by special deflecting coils: coils, frames and coils of lines – special electromagnets placed on the neck of the tube. They are a kind of sighting device that allows you to “fire” any point on the screen by directing an electron beam into it. The line coils make the beam “draw horizontal lines, and the frame coils,” pushing “the beam from line to line, move it vertically.