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Millikan Oil Drop


After the discovery of the electron, scientists knew that this particle was one of the fundamental constituents of matter. Measuring its properties, however, remained a formidable technical challenge. It was Robert Millikan who made the first precision measurement of the electrical charge on the electron. Oil drop experiment, apparatus and the summary of the experiment is described in the given section.


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The Millikan oil drop experiment is a famous experiment to demonstrate the quantized behavior of an electric charge. The idea behind Millikan's experiment to determine the charge on the electron was to attach a number of electrons to an oil drop. Because the mass of the oil drop is much larger than that of an electron, it can be more easily measured.


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From 1909 to 1913, Robert Andrews Millikan (1868-1953) performed a brilliant set of experiments the University of Chicago in which he measured the elementary charge e of the electron and demonstrated the quantized nature of the electronic charge. The apparatus he used is described below. His experimental setup consists of metal plates which is kept at parallely. The charged oil drops (by friction) are allowed to pass through a tiny hole in the upper plate. The droplets are illuminated by using a horizontal light beam. This droplets are viewed by telescope with axis at right angles to the beam. The droplets then appear as shining stars against a dark background and the rate of fall of individual drops can determine.


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The Oil drop experimental apparatus consists of a chamber with metal plates at the top and bottom. These plates could be connected to an electrical potential (voltage). A fine spray of oil droplets was then introduced into the chamber and the droplets were monitored one by one as they fell. Because the droplets were so small, in the absence of a voltage on the plates they soon reached terminal velocity, where the downward force of gravity was balanced by the upward force of air resistance. From the measurement of this velocity, the radius and the mass of the droplet could be determined. At this point, the electrical field was turned on and adjusted so that the droplet remained stationary. The downward force of gravity on the drop was then balanced by the upward electrical force.

Millikan Oil Drop Apparatus

Experiment Summary

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The Oil drop experiment summary is given below:

  • The experiment was performed in 1909 by R.A Millikan.
  • In Millikan's oil drop experiment an electric field is set up between two plates.
  • Oil from an atomizer is sprayed above the top plate and becomes charged by friction.
  • A few drops fall through the hole in the top plate and can be viewed with a telescope.
  • The potential difference between the plates is adjusted till a particular drop is held at rest.
  • At this point the electrical and gravitational forces balance: qE = mg
  • The mass of a drop is determined from the density of the oil and its radius.
  • However, since the drops are very small, their radii must be measured indirectly.
  • The field is switched off and the drops are allowed to fall subject to the viscous drag of the air which is approximately given by, FD = $\gamma$v. where v is the speed of the drop and $\gamma$ is a constant that depends on the radius of the drop and flow resistance of the air.
  • By measuring the terminal speed, one can determine r and then m.
  • Finally the charge is given by q = $\frac{mg}{E}$