The next great step forward in the understanding of atoms was accomplished by John Thomson. Using a cathode ray scope, Thomson determined that all matter, whatever its source, contains particles of the same kind that are much less massive than the atoms of which they form a part. They are now called electrons, although he originally called them corpuscles.
His discovery was the result of an attempt to solve a long-standing controversy regarding the nature of cathode rays, which occur when an electric current is driven through a vessel from which most of the air or other gas has been pumped out.
By applying an improved vacuum technique, Thomson was able to put forward a convincing argument that these rays were composed of particles.
Furthermore, these rays seemed to be composed of the same particles, or corpuscles, regardless of what kind of gas carried the electric discharge or what kinds of metals were used as conductors. From these ideas he developed the idea that atoms are made of negative electrons embedded in a gel of positive charge (a “plum pudding” model).
Thomson’s conclusion that the corpuscles were present in all kinds of matter was strengthened during the next three years, when he found that corpuscles with the same properties could be produced in other ways; e.g., from hot metals. Thomson may be described as “the man who split the atom” for the first time, although “chipped” might be a better word, in view of the size and number of electrons.