Warranting the highly coveted Nobel Prize in 1923, the Millikan oil-drop experiment was a breakthrough in the understanding of the structure of an atom. Robert Millikan came to be known as one of the most brilliant physicists of the twentieth century, as he first attempted the impressive research in 1909, resolute in determining the elementary unit of electric charge. By measuring the effect of an electric field on single drops of evaporating oil, Robert Millikan ingeniously was able to measure the charge of a single electron and then to determine it’s mass. .
By the beginning of the twentieth century, relatively very little was known for certain about the structure of an atom. Whereas many believed that the atom was indivisible, that belief was being critically challenged with the discovery of radioactivity. Even the idea of an elementary unit of electric charge was believed impossible, as many believed that the charge was too infinitely divisible. This changed in 1897, when the British physicist J.J. Thomson was able to determine the ratio of electrical charge to mass for an electron using a cathode-ray tube. This ratio of 1.76*10^8 coulombs per gram, was what enabled Millikan to calculate the mass of a single electron.
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By 1906, Millikan possessed only a modest position of associate physicist at the University, at which time he wrote in his journal, “Although I had for ten years spent on research every hour I could spare from my other pressing duties, by 1906 I knew that I had not yet published results of outstanding importance, and certainly had not attained a position of much distinction as a research physicist.” After that date Millikan came to entirely devote himself in discovering the elementary unit of electric charge. He achieved this remarkable finding by constructing an apparatus in which an atomizer sprayed out a fine mist of oil droplets into an upper chamber, from which some fell through a hole in the upper chamber.