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Millikan's Experiment

We learned from an earlier lesson that J. J. Thomson discovered the electron, and that the electron has a negative charge. But how much of a negative charge does an electron have? What is the charge on an electron "worth"? Is it one? Is it two? One or two what? 

In 1909, an American scientist named Robert A. Millikan (1868-1953) did an experiment that determined the charge of an electron. He used a device that had an atomizer, a spray chamber, two parallel plates, an x-ray source, a variable voltage source, and a microscope.