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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.