Cathode Ray Tube Construction and Working

The Cathode Ray Tube (CRT) have 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 gets sufficient energy to leave the atoms. Alike in photoelectric effect the electrons eject from the surface as a result of particular frequency of light.

Electron Gun

Further, two electrodes are connected with a high voltage battery. Cathode play the role of electron gun. Because electrons emits from the surface of 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 picture.

Deflecting Plates in Cathode Ray Tube

After getting the idea of electron gun and how electrons emit from the cathode surface, i.e. negative plate. Now we will see the behaviour of this electron beam in the presence of 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 as been observed that this electron beam deflected towards the positive plate (that time in 1897 these particles were not recognised by the name of electron, later name given).  

Here, J J Thomson conclude that these particles may have negative charges. The deflection of beam depends on the strength of 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 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 get 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 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 are same. So for a fix current the constant magnetic field will be produced. By varying the current the magnetic field will be 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 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 first figure about it.



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