Cathode Ray Experiment

What is Cathode Ray Tube?

A cathode-ray tube (CRT) is a vacuum tube in which an electron beam, deflected by applied electric or magnetic fields, produces a trace on a fluorescent screen.

The function of the cathode ray tube is to convert an electrical signal into a visual display. Cathode rays or streams of electron particles are quite easy to produce, electrons orbit every atom and move from atom to atom as an electric current.

In a cathode ray tube, electrons are accelerated from one end of the tube to the other using an electric field. When the electrons hit the far end of the tube they give up all the energy they carry due to their speed and this is changed to other forms such as heat. A small amount of energy is transformed into X-rays.

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J. J. Thomson Experiment – The Discovery of Electron

Cathode ray experiment was a result of English physicists named J. J. Thomson experimenting with cathode ray tubes. During his experiment he discovered electron and it is one of the most important discoveries in the history of physics. He was even awarded a Nobel Prize in physics for this discovery and his work on the conduction of electricity in gases.

However, talking about the experiment, J. J. Thomson took a tube made of glass containing two pieces of metal as an electrode. The air inside the chamber was subjected to high voltage and electricity flowing through the air from the negative electrode to the positive electrode.

Apparatus Setup

Cathode Ray Tube Experiment

A Diagram of JJ.Thomson Cathode Ray Tube Experiment showing Electron Beam – A cathode-ray tube (CRT) is a large, sealed glass tube.

The apparatus of the experiment incorporated a tube made of glass containing two pieces of metals at the opposite ends which acted as an electrode. The two metal pieces were connected with an external voltage. The pressure of the gas inside the tube was lowered by evacuating the air.

Procedure of the Experiment

  1. Apparatus as set up by providing a high voltage source and evacuating the air to maintain the low pressure inside the tube.
  2. High voltage is passed to the two metal pieces so as to ionize the air and make it a conductor of electricity.
  3. The electricity starts flowing as the circuit was complete.
  4. To identify the constituents of the ray produced by applying a high voltage to the tube, the dipole was set up as an add-on in the experiment.
  5. The positive pole and negative pole was kept on either side of the discharge ray.
  6. When the dipoles were applied, the ray was repelled by the negative pole and it was deflected towards the positive pole.
  7. This was further confirmed by placing phosphorescent substance at the end of discharge ray. It glows when hit by discharge ray. By carefully observing the places where fluorescence was observed, it was noted that the deflections were towards the positive side. So the constituents of the discharge tube were negatively charged.


After completing the experiment J.J. Thomson concluded that rays were and are basically negatively charged particles present or moving around in a set of a positive charge. This theory further helped physicists in understanding the structure of an atom. And the significant observation that he made was that the characteristics of cathode rays or electrons did not depend on the material of electrodes or the nature of the gas present in the cathode ray tube. All in all, form all this we learn that the electrons are in fact the basic constituent of all the atoms.

Uses of Cathode Ray Tube

  1. Used as a most popular television (TV) display.
  2. X-rays are produced when fast-moving cathode rays are stopped suddenly.
  3. The screen of a cathode ray oscilloscope, the monitor of a computer, are coated with fluorescent substances. When the cathode rays fall of the screen pictures are visible on the screen.

For more information about cathode ray experiment, the discovery of electron or other sub-atomic particles, you can download Byju’s – The learning app. You can also keep visiting the website or subscribe to our YouTube channel for more content.

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