Discovery of the Electron
Scientists worked with electricity long before they understood that current was made of electrons. The cathode tube was a prime example. By switching on some voltage, scientists could make fluorescent streams of electricity travel from the bottom part of a glass tube to the top -- but no one knew how it worked. Some thought the rays were a wave traveling through a mysterious "ether" which they thought permeated all space. Others thought the rays were streams of particles.
J.J. Thomson decided to find out for sure. Thomson was a physics professor at Cambridge University in the UK. He placed cathode tubes in electric and magnetic fields. He knew that these fields will move particles from side to side, but don't have much effect on how a wave moves. In his experiments, the cathode rays bent over to one side, so Thomson knew the cathode rays must be made of some small particle, which he dubbed a "corpuscle."
Thomson initially thought his corpuscles were much too small to be of interest to anyone outside a science lab. However, people quickly realized that electric current was in fact made of moving electrons. Since electricity is the lifeblood of everything from computers to phones to microwaves, the electron turned out to be interesting to just about everybody.
For in-depth information check out the American Institute of Physics' History Center exhibit on the discovery of the electron at http://www.aip.org/history/electron/
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