Before 1909, physicists already knew electrons existed. J. J. Thomson had discovered them in 1897 using cathode ray experiments. But there was still a huge missing piece.
Nobody knew the exact charge carried by a single electron.
That mattered a lot because charge sits at the center of electricity, chemistry, electronics, and atomic physics. If scientists could measure the electron’s charge precisely, they could finally connect electricity to the structure of matter itself.
Millikan receives a check for over $40,000 for winning the Nobel Prize, 1924.
Robert Millikan solved this problem with one of the most famous experiments in physics: the oil drop experiment. He suspended tiny charged oil droplets between electrically charged metal plates and balanced gravity against electric force. By carefully measuring the conditions where a droplet floated motionless in air, he calculated the charge carried by the droplet.
What made the experiment remarkable was not just the number he found. It was the pattern.
Every measured charge turned out to be an exact multiple of one tiny fundamental value.
That value was the elementary charge of the electron.
Today we write it as:
e =1.60217663 × 10-19 coulombs
Modern electronics, quantum physics, chemistry, semiconductor engineering, and even the official definition of the ampere depend on this number.
Why Measuring Electron Charge Was So Difficult
Electrons are unbelievably small.
Their charge is tiny enough that ordinary instruments cannot directly “see” a single electron. In the early 1900s, physicists could observe electrical effects from enormous numbers of electrons moving together, but isolating the charge of one particle was another challenge entirely.
There were several problems:
- Electrons are too light to weigh conventionally
- Their charge is extremely small
- Static electricity effects can interfere with measurements
- Air currents and Brownian motion disturb tiny particles
- Measuring microscopic forces accurately was very hard in 1909
Scientists also did not yet know whether electric charge was continuous or came in discrete packets.
That question sounds strange today because we grow up learning electrons have fixed charge. Back then, it was still possible that charge behaved like a smooth fluid rather than tiny indivisible units.
Millikan’s experiment helped settle that question.
The Basic Idea Behind the Oil Drop Experiment
The core idea was surprisingly elegant.
If a tiny oil droplet carries electric charge, then an electric field can push on it.
Gravity pulls the droplet downward.
Electric force can push it upward.
If those two forces become exactly equal, the droplet will stop moving and float in mid-air.
That balance point reveals the droplet’s charge.
The physics behind it comes from a simple force balance:
qE = mg
Where:
- (q) = electric charge on the droplet
- (E) = electric field strength
- (m) = mass of the droplet
- (g) = gravitational acceleration
If the droplet’s mass and electric field are known, the charge can be calculated.
The hard part was measuring all this accurately for droplets smaller than a grain of dust.
How Millikan Built The Experiment
Millikan and his student Harvey Fletcher designed a chamber containing two horizontal metal plates separated by a small distance.
Millikan's original oil-drop apparatus, c. 1909–1910.
A voltage source created an electric field between the plates.
Tiny oil droplets were sprayed into the chamber using an atomizer, similar in principle to a perfume sprayer.
Some droplets became electrically charged during spraying because of friction and ionization effects. Millikan also used X-rays to ionize air molecules, allowing droplets to gain or lose electrons.
The setup included:
- Two parallel metal plates
- A precisely controlled voltage supply
- A microscope for observing droplets
- An atomizer for creating oil droplets
- A light source to illuminate droplets
- A timing system for measuring droplet motion
Simplified scheme of Millikan’s oil-drop experiment. Credits: Theresa Knott
The droplets were incredibly small, usually only a few micrometers across.
That size mattered for two reasons.
First, tiny droplets fall slowly enough to observe carefully.
Second, very small droplets can be balanced using achievable voltages.
Measuring The Droplet’s Mass
This was one of the cleverest parts of the experiment.
Millikan could not place a microscopic oil droplet on a scale. Instead, he estimated its mass by studying how fast it fell through air.
A falling droplet experiences:
- Gravity pulling downward
- Air resistance pushing upward
Eventually, the droplet reaches terminal velocity, where these forces balance.
Using Stokes’ Law for viscous drag, Millikan could calculate the droplet’s radius from its terminal velocity.
For a spherical droplet moving slowly through air:
F_d = 6 pi η r v
Where:
- (η) = viscosity of air
- (r) = droplet radius
- (v) = terminal velocity
Once the droplet radius was known, its volume and mass could be calculated from the density of oil.
That gave Millikan the gravitational force acting on the droplet.
Balancing Gravity With Electric Force
After calculating the droplet’s mass, Millikan applied voltage between the plates.
This created an electric field.
If the droplet carried negative charge, the electric field could pull it upward. By adjusting the voltage carefully, Millikan could make the droplet:
- rise
- fall slowly
- remain suspended motionless
The floating condition was the most useful because it meant the upward electric force exactly balanced gravity.
At equilibrium: qE = mg
Since electric field strength between parallel plates is: E = V/d
Where:
- (V) = voltage
- (d) = plate separation
Millikan could calculate the droplet’s charge precisely.
The Most Important Discovery: Charge Came In Fixed Units
This is where the experiment became historically revolutionary.
Millikan measured many different droplets.
The charges varied, but not randomly.
Instead, every charge value was always an integer multiple of one smallest quantity.
For example:
- (2e)
- (3e)
- (5e)
- (8e)
But never weird fractions like (2.37e).
That showed electric charge is quantized.
In other words, charge comes in discrete packets.
The smallest recurring value was:
1.6 × 10-19 coulombs
This became known as the elementary charge.
Today we understand that oil droplets gained or lost whole electrons, never fractions of an electron.
At the time, this was powerful evidence that electrons were real physical particles carrying a fixed natural charge.
Why The Experiment Was Harder Than Textbook Diagrams Suggest
School diagrams usually make the oil drop experiment look simple.
In reality, it was extremely delicate.
Several complications had to be handled carefully.
Brownian Motion
Tiny droplets constantly jitter because air molecules collide with them randomly.
This motion, called Brownian motion, introduces measurement uncertainty.
The smaller the droplet, the worse the effect becomes.
Millikan had to average measurements carefully to reduce errors.
Air Viscosity Corrections
Stokes’ Law assumes a continuous fluid.
But very tiny droplets approach the mean free path of air molecules, where classical fluid assumptions start breaking down.
Millikan used correction factors, especially the Cunningham correction, to improve accuracy for microscopic droplets.
Without these corrections, the calculated charge would be wrong.
Evaporation Problems
Water droplets evaporate too quickly.
Millikan switched to oil because oil droplets remain stable longer under observation.
That design choice significantly improved experimental precision.
Ionization Effects
X-rays were used to ionize air molecules, allowing droplets to gain or lose electrons.
This helped Millikan observe changes in charge in discrete steps.
Harvey Fletcher’s Role In The Experiment
For many years, the oil drop experiment was associated almost entirely with Robert Millikan.
But Harvey Fletcher, Millikan’s graduate student, played a major role in the experimental work.
Historical accounts suggest Fletcher helped refine the apparatus and collected much of the data used in the final publication.
There has been historical discussion about whether Fletcher deserved greater recognition. Millikan eventually received the 1923 Nobel Prize in Physics for work including the elementary charge measurement and the photoelectric effect.
Fletcher himself later spoke positively about his collaboration with Millikan, though historians still discuss how credit should be viewed in early 20th century laboratory science.
Did Millikan Select Data?
This became one of the most discussed controversies surrounding the experiment.
Millikan’s notebooks showed that some measurements were excluded from the final published results.
Critics later argued this looked like selective reporting.
Modern historians generally think the situation was more nuanced than outright fraud.
Many rejected data points came from droplets that behaved erratically because of evaporation, instability, contamination, or observational problems. Experimental physicists routinely discard flawed data if there are defensible technical reasons.
Still, the controversy became historically important because it raised questions about experimental judgment, statistical reporting, and scientific transparency.
Interestingly, even with modern reanalysis, Millikan’s final value for electron charge was remarkably close to the accepted value, especially considering the technology available in 1909.
Why This Experiment Changed Physics
The oil drop experiment did far more than measure one number.
It connected several major areas of physics together.
It Confirmed Charge Quantization
Electric charge was shown to exist in discrete units rather than continuous amounts.
That became a foundational idea in atomic physics and quantum theory.
It Helped Determine Avogadro’s Number
By combining the elementary charge with other measurements from electrochemistry, physicists could estimate Avogadro’s number more accurately.
That strengthened the connection between atoms, chemistry, and electricity.
It Improved Atomic Models
Once electron charge became known, scientists could better estimate:
- electron mass
- atomic structure
- electron behavior in matter
It Laid Foundations For Electronics
Every modern electronic system depends on controlled electron flow.
Understanding electron charge was essential for:
- vacuum tubes
- semiconductors
- transistors
- integrated circuits
- digital electronics
Without precise electrical constants, modern electrical engineering would have developed far more slowly.
How Scientists Measure Electron Charge Today
Modern measurements are dramatically more precise than Millikan’s original experiment.
Today, the elementary charge is defined exactly in the International System of Units (SI):
e =1.60217663 × 10-19 coulombs
Since the 2019 SI redefinition, the elementary charge is no longer experimentally measured for defining units. Instead, it is fixed exactly by definition.
Modern experiments involving electron charge use techniques such as:
- single-electron tunneling
- Josephson junctions
- quantum Hall effect measurements
- cryogenic quantum electrical standards
These methods operate at precision levels unimaginable in Millikan’s time.
Still, the oil drop experiment remains one of the clearest demonstrations of quantized charge ever devised.
It is simple enough to explain visually, yet deep enough to connect classical mechanics, electromagnetism, thermodynamics, and quantum physics.
That combination is probably why the experiment still appears in physics classrooms more than a century later.
A Tiny Number That Changed Science
The electron’s charge is one of the smallest measurable quantities in nature, yet it shapes almost every electrical process around us.
Every phone screen, computer chip, battery, radio antenna, and electric motor depends on the movement of electrons carrying this exact amount of charge.
Millikan’s experiment turned something invisible into something measurable.
And that changed physics from a mostly descriptive science into a far more quantitative one.
It gave scientists one of the fundamental constants needed to build modern atomic theory.
Not bad for a few floating drops of oil in a small chamber.
