Cathode Ray Tube Construction and Working

The Cathode Ray Tube (CRT) has three main components, the first one is electron gun, second deflecting plates, and the third one luminescent screen. The CRT principle used in cathode ray oscilloscope (CRO) and Tube based old TVs.

Construction of Cathode Ray Tube:

In this picture of cathode ray tube the first component that is electron gun is shown. When we talk about the electron gun it means how the electrons get produced in the cathode ray tube. In the tube, there is a filament when it glows provide heat to the cathode. As a result, the outer most orbit electrons on surfaces get sufficient energy to leave the atoms. Alike in photoelectric effect, the electrons ejected from the surface as a result of the particular frequency of light.

Electron Gun

Further, two electrodes are connected with a high voltage battery. Cathode plays the role of the electron gun. Because electrons emit from the surface of the cathode as a result of thermal energy. These emitted electrons are attracted by positive terminal i.e. Anode. The acceleration of emitted electrons depends on the potential difference between the cathode and anode.
When you provide sufficient high potential so all electrons reach on the positive plate. Then you observe an electron beam as shown in the below give a picture.

Deflecting Plates in Cathode Ray Tube

After getting the idea of the electron gun and how electrons emit from the cathode surface, i.e. negative plate. Now we will see the behavior of this electron beam in the presence of an electric and magnetic field.

Effect of Electric Field

The figure two right side, observe it carefully what the difference you have seen here as compared to picture one. Actually, we have put an external electric field perpendicular to the beam. And it has been observed that this electron beam deflected towards the positive plate (that time in 1897 these particles were not recognized by the name of the electron, later name given).  

Here, J J Thomson concludes that these particles may have negative charges. The deflection of the beam depends on the strength of the electric field.

So, in the presence of the electric field electron beam deflects with respect to the axis of cathode ray tube at a particular angle. This deflection can be seen on the screen of CRT.


Effect of Magnetic Field

Now, we put the Cathode Ray Tube in the presence of the external magnetic field. Here, we have chosen bar magnet purposely

. Because in the next article when I will demonstrate the experiment, bar magnet are used. Instead of the circular coil known as Helmholtz coils. While here to explain the concept both the ideas are taken so one can’t confuse about it.

By taking two sources of magnetic fields, you will getthe clear picture of the magnetic field effects. Also, both the concepts have their own theoretical formulation. This we will see in the theory part of the Cathode Ray Tube.

Alright, Observe the third figure carefully. What is your observation?  We put the Cathode Ray Tube perpendicular to the external magnetic field. In this case, one observes the deflection of the beam. By varying the position of these bar magnets one can see the variation in the beam on the screen.

In the right-hand side, we have used circular coils in which current is flowing. The number of turn in coils is same. So for a fix current, the constant magnetic field will be produced. By varying the current the magnetic field will vary. Hence, the deflection of the electron beam.

This was about the second component of the Cathode Ray Tube. Now we will see the third component of it that is known as luminescence screen or the fluorescent screen.



Luminescence Screen

The luminescence screen is that part of Cathode ray Tube on which the electron beam strikes and as a result, we observe the bright spot. This is laminated by phosphorous. On this screen, you can see two planes with scale, by which we take observations. You can see in the first figure about it.

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Dr Sushil Kumar

Dr. Sushil Kumar, a physicist, an eminent researcher and a teacher for the benefit of students and fellow physicists alike. Apni Physics is an effort to create a better platform and also to help the students to be able to have content at their hands whenever they want, online. Dr. Sushil continues to upload his lectures and post articles about latest researches in physics, academic, physics education, and also lessons about daily life and how physics define every aspect of our everyday movement and life.

5 thoughts on “Cathode Ray Tube Construction and Working

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