Millikan's Oil Drop Experiment

Jun 20 2026

Aim: To determine the charge of an electron and demonstrate the quantization of electric charge using Millikan's oil-drop apparatus.

Apparatus Required

Millikan's oil drop apparatus (chamber with parallel metal plates)
Atomizer (to spray oil drops)
Microscope with a calibrated eyepiece
High voltage D.C. power supply
Stopwatch
Thermometer
X-ray source or radioactive source (for ionizing air)

Materials / Chemicals Required

Non-volatile oil (e.g., clock oil)

Theory & Principle

The experiment relies on balancing the downward gravitational force with the upward buoyant and electric forces on tiny charged droplets of oil suspended between two metal electrodes. When the drop is stationary, the electrical force ($F_E = qE$) equals the gravitational force minus the buoyant force.

Principle formula: $qE = mg$
Where:
$q$ = charge on the oil drop
$E$ = electric field between the plates ($V/d$)
$m$ = mass of the drop
$g$ = acceleration due to gravity

By measuring the terminal velocity of the falling drop (without electric field) and the rising drop (with electric field), the charge $q$ can be calculated. Millikan found that the charge was always a multiple of a fundamental value: $1.6 \times 10^{-19} C$.

Procedure

Level the Millikan apparatus and focus the viewing microscope.
Spray a fine mist of oil into the upper chamber using the atomizer. A few drops will fall through the small hole into the viewing chamber.
Turn on the X-ray source briefly to ionize the air in the chamber, which imparts an electrical charge to the oil drops.
Observe the drops through the microscope. Select a single, clearly visible drop.
Without Electric Field: Turn off the electric field and measure the time ($t_1$) it takes for the drop to fall a known distance under gravity. Repeat this measurement a few times.
With Electric Field: Apply a voltage across the plates to create an electric field. Adjust the voltage until the drop remains stationary, or measure the time ($t_2$) it takes to rise a known distance.
Record the voltage ($V$) applied and the distance between the plates ($d$).
Calculate the radius and mass of the drop using Stokes' Law from its terminal velocity in free fall.
Calculate the charge on the drop using the balanced forces equation.

Observation Table

S.No.	Distance Traveled (m)	Time of Fall $t_1$ (s)	Voltage Applied (V)	Time of Rise $t_2$ (s)
1
2
3

Result

The electric charge on the oil drops is found to be an integral multiple of a fundamental elementary charge, $e$. The calculated value of the fundamental charge $e$ is approximately $1.602 \times 10^{-19} C$. This proves the quantization of electric charge.

Precautions

The apparatus must be perfectly leveled.
Ensure there are no air currents inside the chamber.
Use a non-volatile oil to prevent the mass of the drop from changing due to evaporation during the experiment.
Focus the microscope carefully to avoid parallax errors while timing the drop.
Do not apply excessively high voltages to avoid sparking between the plates.

Viva Questions & Answers

Q1: Why is oil used in this experiment instead of water?

Water evaporates quickly, which would change the mass and radius of the drop during the experiment, leading to inaccurate calculations. Non-volatile oil maintains a constant mass.

Q2: What is meant by the quantization of charge?

Quantization of charge means that electric charge cannot take any arbitrary continuous value. It always exists as an integral multiple of a basic elementary unit of charge (the charge of an electron, $e$).

Q3: What role do X-rays play in this experiment?

X-rays are used to ionize the air molecules inside the chamber. The oil drops pick up these ions, thereby acquiring an electrical charge necessary for the electric field to exert a force on them.

Q4: What is Stokes' Law?

Stokes' Law gives the frictional drag force $F_d$ acting on a spherical object of radius $r$ moving with velocity $v$ through a fluid with dynamic viscosity $\eta$. Formula: $F_d = 6 \pi \eta r v$.