Robert Millikan and Harvey Fletcher conducted an experiment called Millikan or the oil-drop experiment. The main goal of this experiment was to find out the charge of one electron.

What is the oil drop experiment?

In 1909, Robert Millikan and Harvey Fletcher conducted experiment called the oil-drop experiment at the Ryerson Physical Laboratory at the University of Chicago. The main aim of the experiment was to determine the charge of one electron. Previous experiments, like the one by J.J. Thomson, had the same objective, but could only measure the charge with a predicted order of magnitude. However, Millikan was successful in measuring the charge of a single electron using the oil-drop experiment.

‍

The process of the oil-drop experiment

To conduct the oil-drop experiment, two horizontal metal plates were used with insulating material in between them. The material had four holes in it - three for introducing light and one for viewing through a microscope. A uniform electric field was created between the plates by applying a potential difference.

Next, oil drops were sprayed into a chamber above the plates. The drops got electrically charged due to friction with the nozzle, or an ionising radiation source like an x-ray tube could be used instead. Oil was used instead of water because it evaporates slowly, keeping the mass constant.

‍

To determine the charge of a single electron using the oil-drop experiment, Millikan and Fletcher followed the below steps:

1. Sprayed oil drops into the chamber above the plates.
2. Introduced an electric field between the plates by applying a potential difference.
3. Observed the drops through a microscope and measured their terminal velocity when the electric force balanced force.
4. Calculated the mass of the drop with advanced sedimentation calculations.
5. Adjusted the voltage to balance the drop's weight, and calculated the electric field produced by the applied voltage.
6. Calculated the charge of a single electron using the known mass of the drop and the electric field.

Millikan was able to measure the charge of a single electron to a precision of 1% and increased this value by a factor of 10 within a few years. He also discovered that all charges were multiples of the fundamental electron charge and that electrons could be added or subtracted from the drops at any time.

How did Millikan’s experiments affect photoelectricity?

After conducting the oil-drop experiment, Millikan hoped to invalidate Albert Einstein's theory of the photoelectric effect. He believed that no electrons would be released if the experiment was conducted correctly. However, he ended up proving the theory to be correct, which led to Einstein receiving the Nobel Prize in 1921 for his work on the photoelectric effect.

Despite not achieving his intended outcome, Millikan's experiment was able to determine the charge of a single electron to be -1.60⋅10^-19 C, and that all charges were the fundamental electron charge. Millikan and Fletcher used two horizontal metal plates separated by insulating material with four holes, three for introducing light and one for viewing through a microscope, to conduct the experiment.

Millikan’s Experiment

What did Millikan’s oil-drop experiment reveal?

It determined the charge of a single electron to be -1.6 ⋅ 10 ^-19 and also showed that all charges were multiples of the fundamental electron charge.

What was Millikan’s experiment, and why is it important?

Millikan’s experiment was named the oil-drop experiment, and it is important because it is the first significant measurement of a single electron’s charge.

Which formula was used to calculate a single electron’s charge?

In Millikan’s experiment, the equation q=(m(drop)⋅g)/E=(m(drop)⋅g⋅d)/V was used to calculate a single electron’s charge.

‍