First method for direct measurement of the electric charge of a single electron, originally performed in 1909 by Robert Millikan. He used a microscope to measure the rate of descent of tiny oil droplets directed through the top of a box. By halting the descent of droplets carrying their own electric charge by means of precisely adjusting the voltage between the box's metal top and bottom, he discovered that the electric charges on the drops were all whole-number multiples of a lowest value, the elementary electric charge itself, and thus that electric charge exists in basic natural units.

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