Millikan's

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Millikan's Oil Drop Experiment
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Robert Millikans contribution to the discovery
of atomic structure was preceded by work done by
other scientists.
J. J. Thomson, working/playing with a cathode ray
tube, discovered properties of the electron.
And although Thomson was not able to figure out
the charge of an electron or the mass. He did
experimentally derive the relationship between
the charge and mass of the electron.
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By experimenting with the effect of a magnetic
field as well as an electric field on the
electron J. J. was able to determine the charge
to mass ratio.
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Once the charge to mass ratio had been figured
out, then it would be quite easy to figure out
the mass of the electron if you knew the charge
or vice versa.
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And heres where Robert Millikan steps in.
The following apparatus is the famous Millikans
oil drop experiment
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Mist of oil from oil sprayer
Oil sprayer
Electrically charged plates
Viewing microscope
Source of X-rays
Ionizing radiation
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The oil drops are ionized as they pass through
X-rays
Oil droplets from a perfume mister fall down a
small hole in a metal plate
but still fall to the metal plate.
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The metal plates are charged producing an
electric field
The oil drop is suspended in mid-air by the force
of repulsion from the negatively charged plate
below it.
but still fall to the metal plate.
with the ionizing radiation
Repulsion of like charges
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Results from the calculations of the charges on
the oil drops would look something like this
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Some oil drops have more charge than others.
They are multiples of 1.6 x 10-19.
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OK, put it all together.
J. J. Thomson figured out the charge to mass
ratio of an electron.
1.6 x 10-19
Robert Millikan figured out the charge of an
electron.
Mass of an electron is 9.1 x 10-28 grams