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Cathode Ray Experiment: Summary & Explanation Video

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  • 0:00 What Are Subatomic Particles?
  • 0:54 What Was the Cathode…
  • 2:05 What Did the Results Mean?
  • 2:53 The Discovery of the Electron
  • 3:55 Lesson Summary
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Lesson Transcript
Instructor: Heather Pier

Heather has taught high school and college science courses, and has a master's degree in geography-climatology.

Learn about the Cathode Ray Experiment, one of the most important discoveries of the atomic physics era. Learn how it helped physicist J.J. Thomson's discovery of a previously unidentified subatomic particle.

What Are Subatomic Particles?

For as long as you have been alive, there have been subatomic particles. Subatomic particles are the main parts of an atom: the proton, neutron, and electron. You may have learned about the existence of subatomic particles as early as elementary school. Usually by middle school, everyone knows that protons are positively charged, electrons are negatively charged, and neutrons have no charge. And later on, you would have learned about how each element's atoms contain a unique combination of protons, neutrons, and electrons. But imagine growing up in a time when no one new about subatomic particles, a time when an atom was the simplest part of an element. It was truly a different world, and in the big picture of science, not all that long ago.

What Was the Cathode Ray Experiment?

Picture it: England, 1897. Physicist J.J. Thomson was experimenting with cathode rays. Cathode rays are normally invisible beams of particles that can be contained within vacuum tubes for observation. If cathode rays are combined with fluorescent materials, they can be made visible to the naked eye. Vacuum tubes that can contain cathode rays are called cathode ray tubes.

Cathode ray tubes are not an outdated piece of scientific equipment; in fact, if you are reading this on a non-flat panel desktop computer monitor, or have ever viewed a non-flat screen television, then you have used a modern version of a cathode ray tube before. The tubes are usually made of glass, and contain wires at both ends that can be electrified. The wire allows an electric charge to pass from one end of the tube to the other, sort of like an electric circuit allows charge to flow. In one of Thomson's experiments with cathode ray tubes, he discovered that cathode rays could not travel around solid objects placed in the path of the electric charge but could be focused like a beam of light.

What Did the Results Mean?

What Thomson discovered suggested that cathode rays share properties of both waves (inability to travel around the objects in their way) and particles (the ability to be focused into a beam). Upon further experimentation, he found that if he introduced an electric field into the cathode ray tube set up, he might be able to prove that the cathode rays were carrying particles that were specifically negatively charged. To achieve this, he placed electric plates at both ends of the tube, one negatively charged and one positively charged. Following the concept of opposite charges attracting and like charges repelling, he felt that anything negatively charged in the tube should be repelled away from the negative plate at the end of the tube.

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