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Milliken's Oil Drops and e

Of course, this result revealed nothing about either  e  or  m_e,  just their ratio. But the absolute magnitude of  e  was determined ten years later by Robert A. Millikan, who watched tiny droplets of mineral oil through a microscope: the spherical oil drops, created with an ordinary atomizer (no pun intended), fell through still air in the Earth's gravity at a terminal velocity determined by their weight and the frictional drag of the air, both of which can be calculated from their radius. Now, every once in a while one of the drops would pick up a stray electron and become charged. If the experiment was performed in a vertical electric field of adjustable strength, the charged droplets could be made to "hover" by applying just the right voltage to overcome the force of gravity. Then, knowing the electric field, Millikan was able to calculate the charge.^23.5 The result was   $e = 1.6 \times 10^{-19}$ C,  which meant that the mass of the electron must be really small, namely   $m_e = 9.1 \times 10^{-31}$ kg.