The atom was first discovered in the early 1800's by John Dalton, who acknowledged the "Atomic Theory of Matter", and the "Law of Multiple Proportions", and created the "Table of Atomic Masses". Atoms have been constantly looked at by scientists since then, and are learning more about them every day. Inspired by James Clerk Maxwell, who developed a set of equations expressing the basic law of electricity and magnetism, JJ Thomson looked further into electricity and magnetism and discovered a particle that is contained in atoms, the electron. This is how he came up with the idea
A cathode ray is a stream of electrons, a negatively charged particle two thousand times lighter then a hydrogen atom. Atoms contain electrons; they are near the outside of the atom because they are easily removed. Cathode rays are experimented with, using a cathode ray tube.
A cathode ray tube has a small heater element sealed inside a vacuum, a small positively charged electrode (a solid electric conductor where an electric current enters at one end and leaves at the other), and a negatively charged cathode. There is a fluorescent screen on the other end of the devise. When the element is connected to the cathode, a bright dot appears on the screen. The element is giving out an invisible beam which is attracted to the electrode, passes through it and hits the screen. This beam is the cathode ray. Thomson suggested that a cathode ray is a stream of electrons that is given out by the atom.
At first, Thomson thought that atoms were made up of thousands of electrons to account for their mass. Thus hydrogen would be made up of more than two thousand electrons. If this were true, atoms would have an extremely high negative electric charge, but atoms usually have no charge. Thomson then suggested that atoms did not contain so many electrons, but a number roughly equal to its atomic number. For example, the atom hydrogen contains only one electron, far fewer than the thousands that was originally suggested.
To make the atom neutral, there must be a positive charge somewhere in the atom to balance out the negatively charged electrons. Atoms contain the same number of positively charged particles as they do negative. To express his idea, Thomson invented the "Plum Pudding" model, as it looks a bit like a plum pudding.