It starts with Dalton who in 1803 thought atoms the best way to explain why you can have, for example, carbon-monoxide with a certain amount of carbon and oxygen, and carbon-dioxide with the same amount of carbon and twice the oxygen, but nothing between those two. The multiples were always small whole numbers, as if "individual units" of carbon and oxygen were somehow combining. This idea of individual units of *different masses* (carbon and oxygen) had never been seriously proposed before. http://en.wikipedia.org/wiki/Law_of_multiple_proportions Naturally when ideas progressed to electricity it was assumed that electricity was made up of particles too. They were even named electrons. J.J. Thompson discovered the electron in 1897 when he discovered "cathode rays" could turn a wheel, and be deflected by a magnet showing they were made of negatively charged stuff. http://www.hopkins.k12.mn.us/pages/High/Acad/Sci/Chem/Resources/Notes/atomic_history/thompson.htm The real proof that cathode rays were particles and not just a stream of stuff came in 1909 when millikan was able to show that electric charge always came in whole number multiples of a specific size. This size is the charge on the electron. He did this by elecrically charging oil drops and suspending them using an electric field. This enabled him to measure the field necessary to keep the oil drop from falling (in theory) and hence what the charge on the oil drop must be. http://en.wikipedia.org/wiki/Oil-drop_experiment