Learn about Millikan Oil Drop Experiment in Detail

Millikan’s oil drop experiment was significant in light of the fact that it assisted with deciding the charge of an electron. This was concluded by estimating the electric field strength expected to prevent progressive oil drops from falling. The oil drop experiment was a scientific experiment that was first performed by Robert Andrew Millikan in the year 1827. The experiment includes observing the movement of oil droplets in a fluid.

In this Physics article we are going to discuss more on Millikan’s famous oil drop experiment that led to the discovery of charge and mass of the electron.

Millikan Oil Drop Experiment

In the early 20th century, Robert A. Millikan and Harvey Fetcher made an attempt to measure the electric charge over an electron. The famous Millikan’s oil drop experiment was designed with the same aim and also was a first step towards the target. The experiment took place in the Ryerson Physical Laboratory (RPL) at the University of Chicago. By measuring the deflection produced in the charged oil droplets, Millikan was able to prove the existence of electrons. He was thus able to calculate the actual charge carried by each electron.

Apparatus

The experiment contains a pair of parallel horizontal metal plates. We can easily generate uniform electric fields in the space present between these parallel plates by applying potential differences across the plates. A ring made up of an insulator was used to keep the plates away from each other. Four holes are made into the ring of which three were used for illumination purposes and one for the view through the microscope. A fine droplet of oil was sprayed inside the chamber and allowed to fall on the plate.

Procedure

Let’s have a brief review of the procedure of the experiment and how exactly it takes place. But before that let’s first discuss why the oil has been chosen for the experiment and not the other liquid? This is because, he was searching for something,

• which is a liquid, for spraying purpose on the chamber
• It could easily take the shape of sphere so that he measure its diameter and further use it to calculate the volume
• The substance must be pure so that weight can easily be calculated just by measuring the diameter of the sphere
• Must have extremely low vapour pressure
• and most importantly, it shouldn’t easily evaporate while doing the measurements by any amount.

He found that oil can easily fulfill all these criteria and thus found it ideal for experimental purposes. Now let’s have a look at the procedures of the the experiment:

• Oil is first converted into tiny droplets using an atomizer or spray. Those droplets are then allowed to pass through the holes of the upper plates of the apparatus.
• The oil droplets are then allowed to fall freely and their downward motion is observed carefully using a microscope and after that their terminal velocity is calculated.
• A beam of X-ray is allowed to pass through the gaseous chamber which leads to the ionization of the gasses present inside the chamber. As the oil droplets collide with the gaseous ions present inside the chamber they acquire some electric charges from them.
• Now, the electric field is set up in between the two plates that gave us to the control of motion of charged oil droplets within the field.
• Due to free fall gravity is attracting the charge oil droplets in the downward direction and at the same time electric field applying force in the upward direction over the droplets. The strength of the electric field is regulated in such a manner that the oil droplets reach the point of equilibrium with the gravity.
• Now, finally the charge present over the droplets is calculated at the equilibrium.

Observation and Calculations

At the final stage it is observed that the amount of the charge present over the oil depends upon the strength of the electric field and mass of the oil droplets.

It was observed at the equilibrium we have,

Force in the upward direction = Force in the downward direction —-(1)

Force in the upward direction was due to the effect of applied electric field E = q × E = e × E (in case of the electron)

and, force in the downward direction was due to the gravity = m × g

On substituting the values in equation, (1), we obtained

e × E = m × g

or, e =

Here, e is the charge of electron

E is the measure of intensity of the applied electric field

m = mass of the droplet

In this manner, charge of an electron is measured by Millikan using his experiment. He found that the charge over each drop was some multiple of Coulombs.

Results of Millikan Oil Drop Experiment

Finally, the conclusion was made that the charge present over oil droplets is always an integral multiple of charge of an electron (i.e., e = ). Based upon his experimental evidence he concluded that the charges are always quantised in nature, i,e. in nature, charges are always present in the integral multiple of the fundamental charge of an electron.

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