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Cathode Ray Tube Experiment

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Wilhelm Conrad Roentgen discovered x-rays in 1895. Roentgen was a professor of physics at the university of Wurzberg in Germany. He was doing experiments with a cathode ray tube canned the Hittorf Crooks tube. This glass vacuum tube had an electrical circuit connected to each end.


Roentgen found that some chemicals would fluoresce some distance away from the cathode ray tube. Experiments showed that these rays which had considerable range and penetrating properties, originated from the region where the cathode rays impinged upon the anode end of the tube.

What is a Cathode Ray Tube?

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Cathode Ray Tube Definition

The cathode ray tube is defined to include only glass bulb with phosphor coated on the inside face and with all internal structures including the electron gun. The tube usually has an etched faceplate to reduce glare. In some cases, the yoke is also included as part of the CRT. This is particularly true of color CRTs.

Cathode ray tubes are used in a wide range of devices including cathode ray oscilloscopes, electron microscopes and television sets. The cathode ray oscilloscope is used to view electrical signals or waveforms.

JJ Thomson Cathode Ray Tube

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  1. In 1897 JJ.Thomson devised an experiment with cathode rays that resulted in the discovery of the electron.
  2. Up to this time it was thought that the hydrogen atom was the smallest particle in existence. Thomson demonstrated that electrons comprising cathode rays were nearly 2,000 times smaller in mas than the lightest known particle the hydrogen ion.
  3. When a high voltage is placed across a pair of plates, they become charged relative to each other. The positively charged plate is the anode and the negatively charged plate the cathode.
  4. Electrons pass from the surface of the cathode and accelerate toward the oppositely charged anode. The anode absorb many electrons, but is the anode has slits some electrons will pass through.
  5. The electron travel into an evacuated tube, where they move in a straight line until striking a fluorescent screen. this screen is coated with a chemical that glows when electrons strike it.

J.J. Thomson Cathode Ray tube Experiment is shown below.

J.J. Thomson Cathode Ray Tube

William Crookes Cathode Ray Tube

Sir William Crookes was the first scientist who designed the discharge tube which was called the Crookes discharge tube or Cathode Ray Tube. It is a long glass tube having two metal plates connected to the oppositely charged poles of a battery. The pressure inside the discharge tube can be adjusted by means of an exhaust pump.

William Crookes Cathode Ray Tube

This discharge tube was later slightly modified by J.J.Thomson. When high voltage was applied between the cathode and the anode with a small hole at the center of a partially evacuated tube at a pressure of 0.01mm of Hg, a bright spot of light was formed on the zinc sulfide screen kept at the opposite end of the discharge tube. This was caused by the rays which originated from the cathode called cathode rays.

Cathode Ray Tube Diagram

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A cathode ray tube consists of three main components.
  1. a fluorescent screen
  2. an electron gun and
  3. a system of deflection plates
The cathode ray tube amusement device is shown below.

Cathode Ray Tube Amusement Device
Cathode Ray Tube Diagram

Cathode Ray Tube Experiment Summary

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Thomson experiment proving cathode rays had electric charge. The cathode C and anode A are as in any cathode ray tube. The rays pass through a slit in the anode and a second slit B is interposed in order to make the beam thinner and eliminate rays that were not going straight.
  1. Cathode rays are found to possess the following properties.
  2. They are ejected perpendicular to the surface of the cathode and travel in a straight line due to the intense electric field present near the cathode.
  3. They are deflected by the electric as well as the magnetic field indicating that these rays carry a charge. The direction of deflection indicates that their charge is negative.
  4. Certain substances become fluorescent when put in the path of cathode rays. They are therefore used to detect these rays.
  5. Cathode rays cast shadows of the solids placed in their path.
  6. Cathode rays possess high energy and can make a small fly wheel rotate by imparting their kinetic energy to the fly-wheel.
  7. The nature of the cathode rays is independent of the gas present in the discharge tube.