Thomson discovered that cathode rays are really electrons.
One day Thomson found was that when he connected these big long
hollow cathode ray tubes to batteries, a beam of light would go from one
end to another. Since he had a lot of time on his hands, he decided to
figure out what the deal was with the light. After all, if there was
nothing in the tube to start with, where’d the light come from? He
figured, it must come from the electrodes – since the electrodes were
made of atoms, the atoms must somehow be coming apart.
Among other things, Thomson got a magnet and held it near the beam.
When he did this, he found that the beam would bend toward the positive
side of the magnet and away from the negative side. From this, he
figured that the beam must contain very small particles from the atom
and that they must have negative charge.
This led directly to his “plum pudding” model of the atom, named
after a dessert that nobody has ever heard of. Think of a chocolate
chip brownie, instead. His idea was that the dough in the chocolate
chip brownie made up most of the atom and that it had positive charge.
The chips represented the little tiny bits of negative charge that made
up the light he was messing around with – unlike the dough, they could
leave the atom if you gave them a shove (with a battery, for example).
J.J Thompson, by carrying out a series of experiments with a cathode
ray, realized that electric charge is composed of many smaller
particles; particles that are also smaller than atoms. The experiment
showed that these particles are mobile; therefore, he proposed the idea
that electrons are mobile in the atom.
