Thomson created the cathode ray. The cathode ray is a tube that when a high voltage current is sent through it and the atoms hit the outside of the tube it glows. In this way, Thomson was able to test the polarity of the atoms by placing a magnet next to eh cathode ray. He found out that they were almost all negative, or beta particles. He also determined the charge to mass ratio and came up with the idea that an atom was like plum pudding; mostly pudding(positive 'goo') but every once in a while you get a chunk of plum (electrons).
Significance of Contribution
He helped Milikan determine the charge of an electron later.
He was knighted in 1908 by King Edward VII.