Electron
Year of Discovery: 1897
What Is It? The first subatomic particle ever discovered.
Who Discovered It? J. J. Thomson
Why Is This One of the 100 Greatest?
Atoms had never been seen. Defined as the smallest particles possible and the basic
building blocks of all matter, they were invisibly small—in the late nineteenth century still
more theoretical than real. How could someone claim to have found something smaller?
How could particles get any smaller?
Thomson discovered the electron and proved that it existed—without ever being able
to see or isolate one. Electrons were the first subatomic particles to be discovered, the first
particle of matter identified that was smaller than an atom. This discovery also finally provided some physical proof of, and description of, the basic unit that carried electricity.
Thomson’s experiments and discovery began a new field of science—particle physics.
How Was It Discovered?
He was born Joseph John Thomson in December, 1856, in Manchester, England. By
age 11 he had dropped his first names and used only his initials, J. J. Thomson began engineering studies at age 14 at Owens College and later brought a math and engineering background to the study of physics. In 1884 he was appointed to chair Cambridge’s famed
Cavendish physics lab. Thirteen years later and still at Cavendish, Thomson conducted the
experiment that discovered the electron.
Cathode rays were discovered by German Julius Plucker in 1856. However, scientists
couldn’t agree on what cathode rays were. A great controversy boiled: were they waves or
were they particles? Science’s greatest minds argued back and forth.
In 1896 Thomson decided to design experiments that would settle this dispute. He
built a cathode ray tube and fired its mysterious rays at a metal plate. The plate picked up a
negative charge. This proved that cathode rays had to carry a negative charge. Next, he confirmed with a fluorescent-coated ruler that a magnetic field would deflect cathode rays.
(Others had conducted this experiment.)
Thomson attached thin metal plates inside his cathode ray tube to a battery and showed
that an electrical field could also deflect cathode rays. (The spot that lit up on his fluorescent
ruler shifted when he connected the battery.)
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