Millikan Oil Drop


In 1909, Robert A. Millikan and Harvey Fletcher conducted the oil drop experiment to measure the elementary electric charge, providing crucial insights into the quantization of electric charge and the properties of electrons.


The Millikan Oil Drop Experiment aimed to determine the fundamental unit of electric charge by observing the behavior of charged oil droplets in an electric field. This experiment played a pivotal role in advancing our understanding of the structure of matter and the nature of electric forces.

Experiment Details

Millikan’s setup consisted of a closed chamber filled with air, with two parallel metal plates arranged horizontally. The upper plate was connected to a voltage source, generating an electric field between the plates. Fine droplets of oil were sprayed into the chamber through a small hole in the upper plate.

The oil droplets introduced into the chamber were initially electrically neutral. As some droplets passed through the opening in the upper plate, they entered the space between the plates.

Observation: Using a microscope and illuminating the chamber with a light source, Millikan observed the motion of the oil droplets against a dark background. He carefully monitored their behavior in response to the electric field.

Electric Field and Gravity:Under the influence of the electric field, certain droplets acquired a net electric charge by capturing electrons from the surrounding air molecules. This charge caused the droplets to experience an electric force opposing gravity.

Equilibrium: Millikan adjusted the strength of the electric field until the electric force on the droplets precisely counterbalanced the force of gravity, causing the droplets to remain suspended in mid-air. By measuring the electric field strength necessary for this equilibrium and knowing the mass of the droplets, Millikan could calculate the electric charge on each droplet.

Results and Significance

Through meticulous measurements and calculations, Millikan discovered that the electric charge on the droplets was always a multiple of a fundamental unit of charge. This observation led to the identification of the charge of a single electron, establishing the quantization of electric charge.


Millikan’s Oil Drop Experiment provided the first direct measurement of the charge of individual electrons, confirming the quantized nature of electric charge. This landmark experiment significantly advanced our understanding of the fundamental properties of matter and earned Millikan the Nobel Prize in Physics in 1923.