Robert Millikan and Harvey Fletcher sprayed fine oil droplets into a chamber between two charged plates in 1909 and balanced electric force against gravity to measure the electric charge carried by each drop. Every value they measured turned out to be a small whole number multiple of a single base amount, never a fraction of it, showing that electric charge comes in discrete units rather than a continuous range. That base amount is the elementary charge carried by a single electron, and the finding that charge is quantized became a foundational piece of evidence behind quantum theory.
Facts
Disputed
Key FindingEvery droplet's measured charge was a small whole-number multiple of one base value, about 1.5924(17) times ten to the minus nineteen coulombs, showing electric charge is quantized and providing an early precise measurement of the elementary charge. 1 Sourced to the subject's own accountMillikan's student and uncredited co-experimenter Harvey Fletcher, and later historians of science including Gerald Holton, raised documented disputes over authorship credit and over whether Millikan selectively excluded droplet measurements from the published dataset; David Goodstein's later investigation concluded the exclusions were of incomplete observation series rather than unfavourable results. See the paired Dissent record on this fact. YearSourced to the subject's own account MethodSourced to the subject's own accountMillikan and Fletcher sprayed a fine mist of oil droplets into a chamber between charged plates, timed a droplet's fall with the electric field off to find its radius, then applied a voltage to balance the droplet in equilibrium and calculated its electric charge from the balance of gravitational and electrical forces. 1 LocationSourced to the subject's own accountRyerson Physical Laboratory, University of Chicago 1 Cross-Tradition Connections
Sources
1. Oil Drop Experiment (Wikipedia)
WikipediaProcedure sectionQuote, Procedure section
with zero applied electric field, the velocity of a falling droplet was measured. At terminal velocity, the drag force equals the gravitational force.
1. Oil Drop Experiment (Wikipedia)
WikipediaResults sectionQuote, Results section
they confirmed that the charges were all small integer multiples of a certain base value, which was found to be 1.5924(17)*10^-19 C
Dissenting Readings (1 dissenting reading)
Key Finding
Historian of science Gerald Holton questioned whether Millikan selectively excluded droplet measurements from the published data; a later investigation by David Goodstein concluded the excluded series were incomplete rather than unfavourable. Separately, Millikan's student and co-experimenter Harvey Fletcher recorded, in an account published after both men's deaths, that Millikan required him to relinquish co-authorship on the paper as a condition of his PhD. Neither dispute touches the underlying finding (that electric charge is quantized in units of the elementary charge), only the historiography of how the result was produced and credited.
A dissenting reading, from Gerald Holton; Harvey Fletcher (posthumous account)Oil Drop Experiment (Wikipedia), Wikipedia
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