The experimental apparatus set up by Thomson in 1897. In his apparatus, Thomson coupled two metal plates, which functioned as positive and negative electrodes, in a glass tube whose walls were coated with fluorescent material. Inside the tube, he injected rarefied gas.
It was through this experiment that Thomson tried to apply a large voltage to the terminals of the plates and noticed that a glow illuminated the wall opposite the negative electrode. The explanation for this brightness was the emission of particles, by the negative electrode, which focused on the fluorescent material. These particles were attracted to positively charged plates; and repelled by others, which are negatively charged.
Thomson also studied particles emitted by metallic surfaces illuminated by ultraviolet light (photoelectric effect) and by glowing filaments (thermionic effect). He concluded that, although these were different situations, these particles were identical to those emitted by the negative electrode in the tube of his experiment and came from inside the atom.
These subatomic particles, which were called electrons , have a new property: they are electrically charged and repel each other, overcoming the gravitational force of attraction that exists between their masses.
At the end of the 19th century, scientists carried out numerous experiments to confirm the existence of electrons. Therefore, contrary to what Democritus thought, the atom was divisible. Thomson proposed, in 1898, a new model, according to which the atom would be a massive sphere whose mass practically corresponded to the total mass of the atom itself.
In this sphere would be electrons with a negative charge. As the atom was electrically neutral, Thomson stated that the matter that made up the sphere was positively charged.