Before Thompson and his Cathode Ray Tube Experiment it was believed that atoms where the basic units of matter; however, in 1897 Thompson was the first to suggest that the fundamental unit was 1000X smaller than the atom by discovering the subatomic particle, the electron, with this experiment.
The Cathode Ray Tube
Cathode Rays had been observed as early as the 1700’s, but no one knew exactly what they where???
Are they: Waves of light or Waves of particles?
This was the big question stumping scientists back in the late 17th century, that is until English Physicist JJ Thompson in 1894 was able to prove that:
These rays don’t travel as fast as light so they are more likely to be composed of charged particles
Then JJ created a new and improved cathode ray tube, and conducted a series of experiments on these cathode rays. Using theories of ElectroMagnetism Thompson was able to determine the charge to mass ratio of these particles, and conclude that these particles were really really small -about a 1000x smaller than the smallest known atom Hydrogen- and negatively charged.
This was huge because up until know it was thought that atoms where the basic units of matter, but how can this be when Thompson just proved that particles of matter are existing like a 1000x smaller than the smallest atom.
So Thompson expanded upon Dalton’s Atomic Theory and proposed a new Atomic Model…
The Plum Pudding Model
Thompson, with his limited understanding of the nature of the atom created a theoretical atomic model known as The Plum Pudding Model; which, included his latest discovery, the negatively charged subatomic particle, the electron. Thompson envisioned his electrons as little negatively charged “raisins” surrounded by a soup of positive charge to balance the electron’s negative charge.
Think of the electron’s as the “raisins” that are enclosed by the positively charged “pudding cake”
Thompson’s Atomic Model has since been disproven and expanded upon, but you can’t blame the guy for trying.