page_title: Oil drop experiment
section_title: Background
hierarchical_section_title: Oil drop experiment / Background
caption_attribution_description: Portrait of physicist Robert Millikan
context_page_description: The oil drop experiment was performed by Robert A. Millikan and Harvey Fletcher in 1909 to measure the elementary electric charge. The experiment took place in the Ryerson Physical Laboratory at the University of Chicago. Millikan received the Nobel Prize in Physics in 1923.
  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 and was ionized by an x-ray, making them negatively charged. 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. 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.
context_section_description: 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.
  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 (1.592×10⁻¹⁹ 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 times smaller than 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.
  Aside from the measurement, the beauty of the oil drop experiment is that it is a simple, elegant hands-on demonstration that charge is quantized. 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.
  In the last two decades, several computer-automated experiments have been conducted to search for isolated fractionally charged particles. As of 2015, no evidence for fractional charge particles has been found after measuring over 100 million drops.
caption: Robert A. Millikan in 1891
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