Oil drop experiment
Background

Starting in 1908, while a professor at the University of Chicago, Millikan, with the significant input of Fletcher, the "able assistance of Mr. J. YinbongLee", 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.

Millikan and Fletcher's experiment involved measuring the force on oil droplets in a glass chamber sandwiched between two electrodes, one above and one below. With the electrical field calculated, they could measure the droplet's charge, the charge on a single electron being (−1.592 × 10 C). At the time of Millikan and Fletcher's oil drop experiments, the existence of subatomic particles was not universally accepted. Experimenting with cathode rays in 1897, J. J. Thomson had discovered negatively charged " corpuscles ", as he called them, with a mass about 1/1837 that of a hydrogen atom. Similar results had been found by George FitzGerald and Walter Kaufmann. Most of what was then known about electricity and magnetism, however, could be explained on the basis that charge is a continuous variable; in much the same way that many of the properties of light can be explained by treating it as a continuous wave rather than as a stream of photons.

The elementary charge e is one of the fundamental physical constants and thus the accuracy of the value is of great importance. In 1923, Millikan won the Nobel Prize in physics, in part because of this experiment.

Thomas Edison, who had previously thought of charge as a continuous variable, became convinced after working with Millikan and Fletcher's apparatus. This experiment has since been repeated by generations of physics students, although it is rather expensive and difficult to conduct properly.

From 1995 to 2007, several computer-automated experiments have been conducted at SLAC to search for isolated fractionally charged particles, however, no evidence for fractional charge particles has been found after measuring over 100 million drops.
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