Milliken's Oil Drop Experiment

 The Millikens Oil Drop Experiment was an experiment performed by Robert A. Millikan and Harvey Fletcher in 1909 to measure the charge of an electron. This experiment proved to be very crucial in the physics community. In the experiment, Milliken allowed charged tiny oil droplets to pass through a hole into an electric field. By varying the strength of electric field the charge over an oil droplet was calculated, which always came as an integral value of ‘e.’

Millikan Oil Drop Experiment

Apparatus of the experiment

The apparatus of the experiment was constructed by Milliken and Fletcher. It incorporated two metal plates held at a distance by an insulated rod. There were four holes in the plate, out of which three were there to allow light to pass through them and one was there to allow viewing through the microscope.

Ordinary oil wasn’t used for the experiment as it would evaporate by the heat of light and so could cause error in Millikens Oil Drop Experiment. So, the oil that is generally used in vacuum apparatus which is of low vapour pressure was used.


  • Oil is passed through the atomizer from where it came in the form of tiny droplets. The pass the droplets through the holes present in upper plate of the apparatus.
  • The downward motions of droplets are observed through a microscope and the mass of oil droplets, then measure their terminal velocity.
  • The air inside the chamber is ionized by passing a beam of X-rays through it. The electrical charge on these oil droplets are acquired by collisions with gaseous ions produced by ionization of air.
  • The electric field is setup between the two plates and so the motion of charged oil droplets can be affected by the electric field.
  • Gravity attracts the oil in downward direction and electric field pushes the charge upward. The strength of the electric field is regulated so that the oil droplet reaches an equilibrium position with gravity.
  • The charge over droplet is calculated at equilibrium, which is dependent on the strength of electric field and mass of droplet.


The charge over any oil droplet is always an integral value of e (1.6 x 10-19). Hence the conclusion of  Millikens Oil Drop Experiment is that the charge is said to be quantized, i.e. the charge on any particle will always be an integral multiple of e.

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