Atoms had never been seen before. 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?
J. J. 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.
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.)
Finally, Thomson built a new cathode ray tube with a thin slit through a metal plate. Cathode rays were channeled through this narrow slit. Beyond that metal plate he added a magnetic field to deflect cathode rays in one direction, followed by an electric field that would deflect them back in the other direction.
Thomson knew the force these two fields created. Once he measured the amount of deflection (change of direction) each force created in the stream of cathode rays, he could calculate the mass of the particles in this cathode ray stream. That would finally solve the mystery by identifying the specific particles.
He ran his experiment and didn’t believe his results. The ratio of electric charge to particle mass was way too big, and that meant that the mass of these particles had to be much smaller than any known particle.
He repeated the experiment a hundred times. He ripped apart and rebuilt each piece of equipment. The results were always the same. The mass of this particle had to be less than
1/1000 of the mass of a proton (a hydrogen atom), one thousand times smaller than the smallest atom, supposedly the smallest possible particle.
Thomson had discovered a new particle, the first subatomic particle. It took hundreds of demonstrations and several detailed articles before anyone believed that his new particles existed.
In 1891 Irish physicist George Stoney had named the fundamental unit (particle) of electricity the “electron” without having any idea what that particle was like. Thomson decided to use Stoney’s name (electron) for his new particle since it carried electrical current. In 1898 a Frenchman named Bequerel found photographic proof of the existence of subatomic particles to confirm Thomson’s discovery.
If an electron weighed the same as a dime, a proton would weigh the same as a gallon of milk.
