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Wednesday, 30 April 2008 00:49

Electron discovered April 30, 1897, by Joseph John Thomson

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Of course electrons have been zipping around before we knew about them. However, 111 years ago, Joseph John Thomson first announced that he had discovered the existence of electrons, which he named  "corpuscles,” or small bodies.

At the time, scientists thought that the atom was the smallest indivisible part of matter that could exist. So, Thomson’s announcement, at the Royal Institution (of Great Britain) Friday Evening Discourse, on April 30, 1897, made most scientists a bit skeptical as to his startling news.

In the end, the electron was the first elementary particle to be discovered.

The word “electron” comes from the Greek word for “amber,” which was used by the ancient Greeks to produce an electrical charge by rubbing it onto fur, for instance, which produced a spark when brought in close proximity with a grounded object.

Today, the electron is a fundamental subatomic particle that carries a negative electrical charge. Electrons, along with protons and neutrons, make up atoms.

English experimental physicist Joseph John “J.J.” Thomson (1856-1940) was the director of the Cavendish Laboratory (Department of Physics), at the University of Cambridge, England, at the time of his announcement.

Upon his discovery of the electron, Thomson stated, "Could anything at first sight seem more impractical than a body which is so small that its mass is an insignificant fraction of the mass of an atom of hydrogen?" [AIP: “The Discovery of Fission”]

Thomson discovered the electron, a subatomic particle, through a series of experiments with cathode rays and cathode ray tubes (CRTs), which is a sealed glass cylinder with two electrodes at each end that are separated by a vacuum. Cathode rays are produced when a voltage is applied across the electrodes, which causes the tube to visibly glow.

Find out more about his experiments with CRTs and the discovery of electrons, and his later Nobel Prize. Please continue on with the next page.




Through his experiments, Thomson found that when he applied a magnet (an object containing the property of magnetism—a property of attraction to iron that is inherent in magnets and materials that are induced to have an attraction to iron by introducing a moving electric charge or current through them) to the cathode rays and bent their direction of motion, its negative charge was carried along with the bent rays.

He concluded that the negative charge does not separate from the rays when magnetism is applied to it.

In addition, Thomson conducted experiments on whether cathode rays could be deflected by an electrical field. He constructed a cathode ray tube that had nearly a perfect vacuum (space devoid of matter, like the vacuum of outer space) inside.

He coated one end of the tube with phosphorescent paint. He found that the cathode rays were bend when an electrical field was applied to them; specifically, in a direction that showed the particles had a negative charge. Thus, Thomson concluded that these cathode rays were composed of negatively charged particles, which he called “corpuscles.”

Thomson also calculated the mass-to-charge ratio of the electrons, what he called corpuscles. He measured the amount of deflection brought about them by the application of a magnetic field, and the amount of energy that they carried.

He found the mass-to-charge ratio of the electron to be over one thousand times smaller than that of a hydrogen ion (H+). He concluded that electrons are either very highly charged particles or very light in mass.

Although scientists were initially skeptical of his statements in 1897, he was subsequently awarded the Nobel Prize in Physics in 1906 for his discovery of the electron; specifically, "in recognition of the great merits of his theoretical and experimental investigations on the conduction of electricity by gases."

Along with discovering the electron, Thomson is also credited with discovering isotopes and the invention of the mass spectrometer.

To learn more about the discovery of the electron, please go to the American Institute of Physics (Center for the History of Physics) website “The Discovery of the Electron.”

Also, to learn more about J.J. Thomson, go to: “Thomson, Joseph John” at ChemistryExplained.com.

NOTE by AUTHOR: Yes, first comment is correct. I switched the ratio of charge and mass. It should be (as it is now) the "mass-to-charge" ratio, not the "charge-to-mass" ratio. Thanks for the good catch!


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