n 1887 J.J. Thomson discovery of the electron was a significant step that led to a much deeper understanding of the microscopic properties of Nature. In this entry, I will discuss the famous Millikan Oil drop experiment which was done in 1909.

But recall that in his characterization of the properties of the electron he could only determine the charge to mass ratio, given by -1.76 times ten to the 11th Coulombs per kilogram. Was this ratio a result of two big values or two small values?

Robert Millikan was able to separate the ratio in order to show that the ratio was that of two small numbers and in this way he was able to extract the elementary charge and the electron mass. Comparing the electron mass to that of the lightest element, Hydrogen, it is found that the mass of an electron is 1800 times smaller. Indeed at that time this was the smallest particle known.

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