Millikans Oil Drop Experiment

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Millikans Oil Drop Experiment

• Dr. Frank Walmsley
• Department of Chemistry
• Trinity University
• San Antonio, TX 78212

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Historical Setting

• Early 1900s
• Rutherford had reported his alpha scattering
  experiments. Structure of atom known but not
  accepted by everyone.
• Charge on an alpha particle known.
• Experiments with cathode rays known including
  fact that negatively charged.

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Historical Setting

• Needed to know charge on an electron.
• Millikan modified experiments tried by others and
  did very careful work.
• Used nonvolatile oil rather than volatile water.
• Corrected equations for rate of fall of drop
  determined this experimentally

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Historical Setting

• Robert Millikan
• Received Nobel Prize for this work
• Work done at University of Chicago
• Became first president of Californa Institute of
  Technology

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Design of the Experiment

• Sketch of apparatus used
• Blue plates charged charge can be varied. Oil
  drops fall through hole in top plate. Xrays place
  electrons on drops. Microscope used for observing
  drops.

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Design of the Experiment

• Photo of actual apparatus used in the
  experiments.

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Design of the Experiments

• Time of fall of oil drop in the absence of field
  measured between two fixed points. Drop falls due
  to gravity.
• Electric field needed to hold drop steady between
  the two plates measured.
• When drop is suspended, force due to gravity
  equals force due to electrical attraction to
  plate.

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Design of the Experiment

• Information needed
• Density of air
• Viscosity of air
• Density of oil
• Equation for rate of fall
• Strength of electric field
• Temperature
• Air Pressure

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Data Treatment

• From Millikans 1911 paper.

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Data Treatment

• Notice that dividing by the smallest of the three
  numbers does not give whole number results.
• 1.996 1.000 2.813
• Millikan had lots of measurements, not just 3
  which is needed in order to find that common
  multiplier.

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Experimental Results

• Showed that the charges were multiples of the
  minimum basic charge of one electron.
• Determined the value of that charge.
• This led to more accurate values for other
  constants such as Avogadros Number.

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Was Millikan Dishonest?

• Millikan did not use all his data points in his
  published articles.
• Millikan said in his articles that he did use all
  points.
• How can both be true?

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Was Millikan Dishonest?

• It is true that Millikan selectively chose points
  to report.
• He discarded drops that were too small because
  they were too affected by bombardment from air
  molecules.
• He discarded drops that were too large because
  they did not obey the equations for rate of fall.
• He discarded drops that had only one measurement
  that is, did not have additional measurements
  with fewer or greater numbers of electrons.

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Was Millikan Dishonest?

• It appears that what he meant (and did not say
  clearly) was
• He used all drops that met his criteria for being
  reliable.
• If he had used all drops, the results would have
  been essentially the same.

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