Do you know someone with an old tube TV? Or did you come across oscilloscopes with a tube screen in physics class? This type of screen is based on the Braun tube, the function of which you will learn more about in this article.
Braun tube: structure and function
The Braun tube uses a beam of electrons to draw images on a fluorescent screen. These are then deflected by electromagnetic fields.
Basically, the Braun tube accelerates electrons using a high voltage on a fluorescent screen to produce a glow there. The beam can diverted be used to display different patterns on the screen.
In the next sections you will learn what the Braun tube looks like in detail and which components are responsible for which process.
Braun tube: sketch
You can see the schematic structure of the tube in the figure.
The Braun tube is built entirely in a vacuum tube. This prevents the electrons from colliding with gas molecules in the air, which would weaken the beam.
Inside this tube is the thermionic cathode on the far left.
thermionic cathode
The hot cathode is a wire to which a heating voltage is applied. It is high enough for the wire to heat up due to its own resistance. It gets so hot that the glow-electric effect Electrons can escape from the heating coil.
A current flow through the wire succeeds in releasing electrons from the filament and bringing them into the vacuum. The emission is comparable to the evaporation of water on the surface of the wire.
Wehnelt cylinder
The Wehnelt cylinder is a cylindrical, conductive metal body. A negative voltage is applied to it relative to the hot cathode. As the cylinder surrounds the cathode, the ejected electrons are focused by it and exit through a hole on the right.
Depending on the applied negative voltage, the electrons are bundled to a greater or lesser extent. This allows the intensity of the electron beam and thus the brightness of the pixel on the screen to be controlled.
anode
The anode has the task of accelerating the electrons that have escaped from the Wehnelt cylinder in the direction of the screen. There is one for that anode voltageat the anode.
Since this is positive compared to the Wehnelt cylinder, the electrons are attracted and accelerated to the anode. The electrons pass through a hole in the middle of the anode and then move on at a constant speed.
baffles
The previously generated electron beam has to be deflected in order to generate images on the screen. Because otherwise the beam just runs straight towards the screen and you would only see a dot in the middle.
The distraction gets through electromagnetic fields generated between the baffles. You can imagine the fields that are generated as with a plate capacitor. They are namely homogeneous electric fields. That is why the voltage applied here is called capacitor voltage. Depending on the applied deflection voltage, the beam is deflected to different degrees.
Different pairs of deflection plates are used to achieve vertical or horizontal deflection.
fluorescent screen
The luminescent screen is a phosphor layer on the inside of the vacuum chamber. When the beam of electrons hits this luminous layer, it lights up where you can see it.
Braun tube: formula and tasks
You now know the different components of the Braun tube and how the electron beam passes through them. You can now calculate the velocity and deflection of the beam if you know the voltages involved.
speed of the electrons
The acceleration curtain here is the same as in a linear accelerator.
For the calculation you equate the kinetic and electrical energy of the electrons, you can see the formulas here:
You can now enter more specific information in the formulas: for the mass m the electron mass, for the charge q the electron charge e and for the voltage U the anode voltage:
Now you change the formula according to the speed v you are looking for after the acceleration process.
So you can calculate the final velocity of the electrons in the electron beam of the Braun tube:
To calculate the speed of the electrons After the acceleration process in the Braun tube, you need the electron mass, the electron charge e and the anode voltage:
To deepen your knowledge, you can take a closer look at the calculation of the speed using an example.
task
Calculate the speed of the electrons in a Braun tube when accelerated with a voltage of 250V.
In addition, the magnitudes of the electron mass and the electron charge are known.
solution
You can use the formula above to calculate speed:
Now plug the given values into the formula. is.
So you know the speed of the electrons that are accelerated towards the screen.
The electrons accelerated to their speed can now be deflected to draw images on the screen. You can take a closer look at this in the next section.
deflection of the electrons
After the electrons have been accelerated, they have in the Braun tube a constant speed. To analyze deflection, first look at the process through a couple of deflection plates.
The following example shows you what the calculation for the distraction looks like.
To calculate the deflection, you can imagine the beam in a coordinate system: The x-coordinate is the movement in the direction of the screen and the y-coordinate is the deflection of the beam by the deflection plates.
Since the acceleration in the x-direction has already been completed, the speed of the movement there is constant. So you can covered trackin X direction for the electrons as follows:
where v is the velocity of the electrons after acceleration.
However, the deflection plates accelerate the electrons in the y-direction with a uniform acceleration. You can determine the distance covered by the electrons in the x-direction using the following formula:
To calculate the acceleration in the y-direction you have to equate the force on the electrons with the force of the electric field of the deflection plates. For this you need the deflection voltage and the distance between the deflection plates d:
Rearrange the formula after the acceleration:
Now you can put the acceleration into the formula for the Deflection in the y-direction deploy:
Now it is possible to calculate the deflection after a certain time t.
However, if you are interested in the deflection at the end of the deflection plates, you need to determine how long it takes the electrons to get to this point. The length covered is l.
Changed by time:
Inserted into the above formula, the deflection in the y-direction results in that at the end of the plates to calculate:
After that, the electrons make an unaccelerated motion in a straight line with length in the x-direction from b until they hit the screen.
In summary, you can calculate the deflection in the x-direction and y-direction using the following formulas:
You now know the components of the Braun tube and can calculate the electron orbit. But how was the tube developed and where is it used? You will find out in the next section.
Braun tube: inventor and history
After the from Thomas Edison (1847–1931) discovered the glow-electric effect, which is used in the Braun tube to generate the electrons for the electron beam Karl Ferdinand Braun (1850–1918) in 1897 the screen of the same name.
With the tube, Braun examined electrical oscillations using the same principle as analog ones oscilloscopes work: A variable voltage on the deflection plates ensures a different deflection on the fluorescent screen.
Also black and white TV pictures can be generated by a tube with two pairs of baffles. The beam is directed onto the screen line by line. Depending on the desired brightness, the voltage on the Wehnelt cylinder is now changed. Since the beam moves quickly across the screen, the course of the beam is no longer visible to the eye, but a coherent image results.
Braun tube Usage: oscilloscope and color TV
analogues oscilloscopes use the Braun tube to display voltage curves. The voltage to be measured at the deflection plates causes a corresponding deflection on the fluorescent screen. A second pair of deflection plates can be used to show the course over time by moving the beam from left to right, so that the course of the applied voltage becomes visible.
The functioning of a color television is a bit more complex, because you don’t just need light and dark like a black and white picture, you also have to display colors.
For this you use the colors red, green and blue, which are attached as small fluorescent dots next to each other on the screen for each pixel.
Now three electron beams are used, one for each color. These beams are directed onto a pixel at the same time and the respective beam lights up its color. The desired color spectrum can then be displayed by combining the three colors.