Millikan Oil Drop Experiment

First Measurement of Fundamental Electric Charge on Electron

Jun 12, 2009

In 1910, physicist, Robert A. Millikan, first measured the charge on the electron, which is the fundamental electric charge, with his classic oil drop experiment.

Physicists know that the charge on an electron is the fundamental unit of electric charge. This amount of charge, 1.6E-19 coulombs, is the smallest amount of charge that can occur in nature. This fundamental electronic charge can not be broken into smaller amounts of charge.

Robert Andrews Millikan first accurately measured the value of the fundamental electric charge with his famous oil drop experiment.

Millikan Oil Drop Experiment

Prior to the Millikan oil drop experiment, J.J. Thomson had measured the charge to mass ratio (e/m) of the electron in 1897. Thomson had been unable to accurately measure the charge on the electron, but he did establish an approximate range of possible values for this charge.

Thomson's experiment used electric charges on a cloud of water. Millikan's experiment was similar, but replaced the cloud of water with individual oil droplets.

With his oil drop experiment, Millikan was able to accurately measure the charge of the electron, e. When combined with the charge to mass ratio, he was able to calculate the mass of the electron.

Description of the Experiment

Millikan used an atomizer to spray fine droplets of oil into his experimental apparatus. In this process the oil droplets also acquired an electric charge from surplus electrons.

The charged oil droplets floated between two metal plates. Millikan connected these plates to a high voltage power supply so that the electrical force between the plates pushed the electrons upward, counteracting the downward gravitational force. Millikan also used a light to illuminate the oil droplets, which he observed through a small telescope with a fine scale to measure the distance the droplets moved.

When Millikan turned the voltage on the metal plates off, the droplets drifted downward. When he turned the voltage back on the droplets slowly drifted upwards. Millikan corrected for the viscous drag on the moving oil droplets and measured the upward and downward speeds for individual oil droplets as they repeatedly moved up and down. For his original experiment, Millikan followed a total of 58 individual oil droplets over a period of 60 days.

Occasionally an oil droplet would change its charge and its upward speed would suddenly change. Comparing the downward velocity and two different upward velocities allowed Millikan to calculate the small change in the electric charge on the oil droplets. This change was always an integer multiple of the fundamental electric charge on a single electron. Plotting the measured changes in charge on his 58 oil droplets and extending the slope of his graphs to the smallest possible charge difference gave Millikan the value of the charge on a single electron.

Millikan's value for the charge on an electron was 1.592E-19 coulombs, which is very close to the modern value of 1.6022E-19 coulombs.

Significance of the Millikan Oil Drop Experiment

In addition to measuring the charge on an electron, Millikan's experiment showed that the charge on a single electron was the smallest possible amount of charge. Millikan proved that electric charge is quantized, as predicted by quantum physics. The total amount of electric charge must always be an integer multiple of this fundamental electric charge. Millikan also used his result to calculate the value of Avogadro's number.

The Millikan oil drop experiment was sufficiently important that Millikan won the 1923 Nobel prize in physics for this work.