<P> The oil drop experiment was performed by Robert A. Millikan and Harvey Fletcher in 1909 to measure the elementary electric charge (the charge of the electron). </P> <P> The experiment entailed observing tiny electrically charged droplets of oil located between two parallel metal surfaces, forming the plates of a capacitor . The plates were oriented horizontally, with one plate above the other . A mist of atomized oil drops was introduced through a small hole in the top plate . First, with zero applied electric field, the velocity of a falling droplet was measured . At terminal velocity, the drag force equals the gravitational force . As both forces depend on the radius in different ways, the radius of the droplet, and therefore the mass and gravitational force, could be determined (using the known density of the oil). Next, a voltage inducing an electric field was applied between the plates and adjusted until the drops were suspended in mechanical equilibrium, indicating that the electrical force and the gravitational force were in balance . Using the known electric field, Millikan and Fletcher could determine the charge on the oil droplet . By repeating the experiment for many droplets, they confirmed that the charges were all small integer multiples of a certain base value, which was found to be 6981159240000000000 ♠ 1.5924 (17) × 10 C, about 0.6% difference from the currently accepted value of 6981160217648699999 ♠ 1.602 176 487 (40) × 10 C. They proposed that this was the magnitude of the negative charge of a single electron . </P> <P> Starting in 1908, while a professor at the University of Chicago, Millikan, with the significant input of Fletcher, and after improving his setup, published his seminal study in 1913 . This remains controversial since papers found after Fletcher's death describe events in which Millikan coerced Fletcher into relinquishing authorship as a condition for receiving his PhD . In return, Millikan used his influence in support of Fletcher's career at Bell Labs . </P>

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