# Today in Science History – December 19 – Robert Millikan and Oil Drops

Robert Andrews Millikan
(1868-1953)

December 19 marks the passing of Robert Millikan. Millikan was an American physicist best known for his famous oil drop experiment.

The purpose of this experiment was to measure the charge of an electron. Millikan and Harvey Fletcher sprayed charged drops of oil between two horizontal charged plates. Gravity pulls the oil drops down while electrostatic forces between the drops and the charged plate pull the drops upwards. As the drops were suspended, they could measure their diameters and calculate their volume. They then used the density of the oil to determine the mass of each drop.

To account for air resistance as the drops fell, they applied Stokes Law to calculate the drag the drop encounters as it falls. This force changes the apparent weight of the drop as it falls through the air. Both forces could be balanced by applying a voltage to the parallel plates. The electrostatic force applied to balance these forces is directly proportional to the charge of the oil drop.

When this experiment is run against multiple oil drops the resulting charges all have a common factor of one unit of electrical charge or the charge of a single electron. Millikan and Fletcher’s value for the charge of an electron was 1.5924(17)×10−19 Coulombs. Today, the accepted value for the charge of an electron is 1.602176487(40)×10−19 Coulombs. The measurement of this fundamental constant earned Millikan the 1923 Nobel Prize in Physics and Harvey Fletcher his doctorate degree.

### Notable Science History Events for December 19

#### 2013 – ESA launches Gaia Telescope.

Artist’s impression of the Gaia Spacecraft ESA

The European Space Agency launched their Gaia spacecraft. Gaia was designed to create a 3D map of nearly one billion stellar objects. The equipment on board would take 70 different observations of each object to determine it’s position, velocity, and parallax over the course of its five-year run. The spacecraft’s instruments have managed to maintain their integrity better than hoped and plans are in place to increase the mission.

#### 2004 – Herbert Charles Brown died.

Brown was an Anglo-American chemist who was awarded half the 1979 Nobel Prize in Chemistry for his development of organoborane chemistry. Organoboranes are organic compounds which have boron and hydrogen together and are used in many transformation reactions.

#### 1961 – Eric Allin Cornell was born.

Cornell is an American physicist who shares the 2001 Nobel Prize in Physics with Carl Wieman and Wolfgang Ketterie for synthesizing the first Bose-Einstein condensate (BEC). A Bose-Einstein condensate is a state of matter where a dilute gas near absolute zero where all particles occupy the lowest quantum state and the quantum effects appear on a macroscopic level. This achievement verified the existence of BEC predicted by Satyendra Bose and Albert Einstein in 1924.

#### 1936 – Juan de la Cierva died.

Juan de la Cierva (1895 – 1936)

Cierva was a Spanish civil engineer who invented the autogyro. An autogyro is an aircraft that uses a rotary wing system to provide lift. The autogyro uses a conventional propeller to pull the aircraft forward which starts the gyro rotating and the aircraft begins to fly. This invention led directly to the invention of the helicopter.

#### 1903 – George Davis Snell was born.

Snell was an American geneticist who discovered the genetic factors involved to determine whether or not a transplant of tissue from one organism to another is possible. This would earn him one-third of the 1980 Nobel Prize in Medicine.

#### 1852 – Albert Abraham Michelson was born.

Albert Abraham Michelson (1852 – 1931)

Michelson was an American physicist best known for his quest to accurately measure the speed of light. He was also the Michelson from the Michelson/Morley experiment to disprove the existence of the aether, the medium of space. He developed many new precision optical devices to refine his measurement of the speed of light and was awarded the 1909 Nobel Prize in Physics for his efforts.

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