Millikan's Oil Drop Experiment

John Dalton in 1804 proposed in his Atomic Theory that atoms cannot be subdivided

However, with the discovery of the electron in 1897, the idea of sub-atomic particles came to light

J. J. Thomson with his series of experiments calculated the Charge to Mass Ratio of an electron

After the discovery of the electron, Thomson continued his experiments on the Cathode Ray Tube

And found the magnitude of Charge to Mass Ratio of an electron to be as above

Another Scientist by the name Robert Andrews Millikan conducted an experiment on oil drops

With his experiment, he found out the charge of an electron

This was the setup of the experiment Millikan performed to determine the charge on an electron

Now let us try to understand Millikan's Oil Drop Experiment

In the experiment, there is an atomizer that sprays oil drops in the form of mist

The drops produced are allowed to pass through a plate of an electric condenser

The air inside the chamber is ionized by the passage of X-Rays

On colliding with these gaseous ions

The oil drops acquire charge

Obviously, due to the mass of each drop (however small it may be), Gravitational Force acts on it

Due to the presence of Electric Field, an Electrostatic Force acts on the oil drops as well

And also, due to a natural friction in air, a Viscous Drag Force acts on it

The rate at which the oil drops fall was measured using a micrometer eyepiece of a telescope

The rate at which these droplets fell was controlled by varying the Electrical Field in the chamber

By neutralizing the gravitational force by the electric force he observed that the electric charge on the droplets was always in the multiples of

Hence the charge is always , where and so on. He observed that this value of was

We can extend this concept to calculate the mass of an electron using Thomson's obtained Charge to Mass Ratio

Revision

Millikan found the charge on an electron through his Oil Drop Experiment

He found the charge on the oil drops as multiples of

The End