Millikan Oil Drop Experiment: Definition, and Examples

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Millikan Oil Drop Experiment Definition

In this experiment, Millikan permitted the charged tiny droplets of oil to pass through a hole into an electric field. Then, by changing the strength of the electric field, the charge over an oil droplet was calculated, which always results as an integral value of ‘e.’

What is Millikan Oil Drop Experiment?

Millikan oil drop experiment is accomplished by Millikan and Harvey Fletcher in 1909 to measure the charge of an electron. This experiment proved to be helpful in the physics community.

Robert Andrews Millikan was a famous American physicist and was awarded the Nobel Prize for Physics in 1923 for his work on the elementary electronic charge and the photoelectric effect.

In 1909, Millikan performed a series of experiments to find the electric charge which is carried by an electron. He initially began his experiment by measuring the path of charged water droplets in an electric field. The results projected that the charge present on the droplets is a simple multiple of the basic electric charge, but the test’s result was not precise enough to be considerable.

To obtain more specific results, in 1910 he performed his famous oil-drop experiment in which he substituted water (which tends to evaporate quickly) with oil, and this experiment is further explained below;

Millikan Oil Drop Experiment Apparatus

The apparatus for the experiment was created by Millikan and Fletcher. It is consists of two metal plates that are held at some distance by an insulated rod. Four holes were made in the plate, out of which three were only allowed to pass the light through them and the fourth one is used to view through the microscope.

Ordinary oil was not used for this experiment as it tends to evaporate by the heat of the light and therefore an error could be caused in Millikan Oil Drop Experiment. So, the oil having low vapor pressure was used, the same that is used in a vacuum apparatus.

• A specific type of oil as mentioned above is sprayed into the chamber, where the drops attain electrical charge.

• The droplets are then allowed to enter the space present between the plates and, as they were charged, they could be effortlessly controlled by altering the voltage across the plates.

• Mainly, the oil drops were allowed to fall between the plates having no electric field. They then rapidly reached terminal velocity due to the presence of friction of the air in the chamber.

• The field was then turned on and it was huge enough, thus some of the drops started to rise. This is because of the presence of upwards electric force, (FE) on them which is greater than the downwards gravitational force, g.

• Millikan’s experiment was actually meant to have the drops to fall at a constant rate. At this constant rate, the gravitational force present on the drop and the force of the electric field or upwards electric force on the drop is equal.

• Millikan then repeated this same experiment for over 150 oil drops out of which he selected 58 oil drops results and then with the help of these observations he determined the highest common factor.

Millikan Oil Drop Experiment Calculation

As mentioned above,

Fup = Fdown

Fup = Q. E

Fdown = m.g

Where, Q represents an electron’s charge

E represents the electric field

m represents the droplet’s mass, and

g represents gravity.

Q⋅E = mg


Q = mg /E

It can be said that an electron charge is measured by Millikan. Millikan stated that all drops had charges that were equal to 1.6x 10-19 C multiples.

Millikan Oil Drop Experiment Conclusion

The charge present on an oil droplet is always equal to an integral value of e (1.6 x 10-19). Hence, the assumption of Millikan’s Oil Drop Experiment displays that the charge is quantized, that is the charge present on any particle is always be an integral multiple of e.

Millikan’s oil drop experiment was a vibrant demonstration of the quantization of charge.

The experiment has since been commonly conducted by many physics undergraduates, though it is quite expensive and to get the precise result is quite difficult.

Millikan Oil Drop Experiment Importance

Millikan’s experiment is quite crucial to study because it establishes the charge over an electron.

Millikan used a simple device in which he adjusted the actions of electric, gravitational, and air drag forces.

With the help of the apparatus, he was successful in estimating the charge on an electron that is equal to 1.60 × 10-19 C.

Reason for Using Oil Drops

Oil drops are used in Millikan oil-drop experiment because oil drops usually retain their mass over some time when it is exposed to higher temperatures. Likewise, he used an atomizer for ultra-fine droplets. Therefore, he preferred oils over water because water changes its state or form at much higher temperatures.

Furthermore, it is extensively known that oil tends to retain the exact volume, omass, and weight. This property of oil enabled him to have a precise measurement of the charge. Other liquids present in nature may separate, disintegrate or evaporate.

Millikan Oil Drop Experiment Citations


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