Wikipedia Deep Dive

Oil drop experiment

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The Experiment That Weighed the Unweighable

In 1909, in a basement laboratory at the University of Chicago, two physicists spent hours staring through a microscope at tiny droplets of oil floating between metal plates. It sounds almost comically mundane. But what Robert Millikan and Harvey Fletcher were doing was nothing less than measuring the weight of electricity itself—or more precisely, determining the charge carried by a single electron.

This was audacious.

At the time, not everyone even believed electrons existed. The idea that electricity came in discrete, indivisible packets rather than flowing like a continuous fluid was still controversial. Sure, J.J. Thomson had discovered something he called "corpuscles" in cathode rays back in 1897—tiny negatively charged particles about two thousand times lighter than a hydrogen atom. But many physicists remained skeptical. Most electrical phenomena could be explained perfectly well by treating charge as a continuous variable, just as you can explain most properties of light without ever invoking photons.

Millikan and Fletcher's experiment would change that. It would prove, beyond reasonable doubt, that electric charge comes in fundamental units—that there is a smallest possible amount of charge, and that every charged object in the universe carries some whole-number multiple of it.

The Setup: Suspending Oil in Midair

The apparatus was elegant in its simplicity. Two horizontal metal plates, one above the other, separated by a ring of insulating material. By connecting these plates to a voltage source, you create a uniform electric field in the space between them—think of it as an invisible force pushing upward on anything with a negative charge.

Above the top plate sat a chamber where oil could be sprayed into a fine mist. The experimenters used a special low-vapor-pressure oil, the kind normally found in vacuum equipment. This mattered because ordinary oil would slowly evaporate under the heat of the bright lamp illuminating the chamber, and if your droplet is changing mass during the experiment, your measurements become meaningless.

Some of these tiny oil droplets picked up electric charge as they were sprayed through the nozzle—friction does that. Alternatively, the experimenters could blast the chamber with X-rays to ionize the air and give the droplets a charge.

Then came the magic trick. With the electric field turned off, a droplet would fall under gravity, quickly reaching a terminal velocity where air resistance exactly balanced its weight. But turn on the field, adjust the voltage just right, and you could make a charged droplet hover in midair. The electrical force pulling it up would exactly cancel the gravitational force pulling it down.

A droplet suspended in space, defying gravity through pure electromagnetism. There's something almost mystical about it.

The Mathematics of Falling and Floating

Here's where physics gets clever. By watching how fast a droplet falls with the field off, you can figure out its size.

This works because of something called Stokes' Law, which describes how small spheres move through viscous fluids. The drag force on a falling droplet depends on both its velocity and its radius. But the gravitational force pulling it down depends only on its radius (specifically, on the cube of its radius, since that determines its volume and thus its mass). Since these two forces must be equal at terminal velocity, and since you can measure the velocity, you can solve for the radius.

Once you know the radius, you know the mass. Once you know the mass, you know the gravitational force. And once you know the gravitational force, you can figure out what electrical force is needed to exactly counteract it.

The electric force on a charged particle is simply its charge multiplied by the strength of the electric field. The field strength is just the voltage divided by the distance between the plates. So if you know the voltage required to suspend a droplet, and you know the gravitational force that voltage is counteracting, you can calculate the charge.

The beautiful part came when Millikan and Fletcher repeated this process for hundreds of different droplets, each with a different amount of charge. What they found was remarkable: the charges weren't random. Every single charge they measured was a whole-number multiple of a single base value.

One droplet might carry exactly this base charge. Another might carry exactly twice as much. A third might carry exactly five times as much. But no droplet ever carried, say, one-and-a-half times the base charge, or two-and-three-quarters times.

This could only mean one thing: electric charge is quantized. It comes in discrete, indivisible units. And that base value—about 1.6 times ten to the negative nineteenth coulombs—is the charge of a single electron.

How Small Is an Electron's Charge?

Let's put this number in perspective. A coulomb is the standard unit of electric charge, roughly the amount of charge that flows through a 100-watt lightbulb in one second. The charge on a single electron is about 0.00000000000000000016 coulombs.

That's a decimal point followed by eighteen zeros and then a 16.

To accumulate one coulomb of charge, you'd need to gather about six quintillion electrons—that's a 6 followed by eighteen zeros. When you flip a light switch and current flows through the wire, roughly six quintillion electrons are passing through every second.

The precision of Millikan's measurement was extraordinary for its time. His published value of 1.5924 times ten to the negative nineteenth coulombs differs from today's accepted value by less than one percent. This was good enough to earn him the Nobel Prize in Physics in 1923.

The Controversy That Wouldn't Die

But Millikan's Nobel came with an asterisk that historians are still arguing about a century later.

The first controversy involves Harvey Fletcher, the graduate student who worked alongside Millikan on the experiment. Papers discovered after Fletcher's death in 1981 tell a troubling story. According to these documents, Millikan pressured Fletcher into giving up authorship credit on the crucial papers as a condition for receiving his doctorate. In exchange, Millikan reportedly used his considerable influence to advance Fletcher's career at Bell Laboratories.

Fletcher went on to become a distinguished physicist in his own right—he's remembered as the "father of stereophonic sound" for his work on audio engineering. But for decades, the oil drop experiment bore only Millikan's name.

The second controversy is more scientifically interesting. In 1978, historian Gerald Holton examined Millikan's laboratory notebooks and noticed something strange: Millikan had recorded far more measurements than he included in his published results. Some data points appeared to have been excluded without clear justification.

Was Millikan cherry-picking his data? Throwing out measurements that didn't fit his theory?

This set off a debate that continues among historians and philosophers of science. Allan Franklin, a physicist and philosopher at the University of Colorado, argued that Millikan's exclusions didn't significantly change his final value for the electron's charge—they just made his statistical error bars smaller. In other words, Millikan may have been guilty of making his results look more precise than they actually were, but not of actually getting the wrong answer.

David Goodstein, examining Millikan's notebooks more carefully, offered a different defense. He found that Millikan had clearly noted which measurements constituted a "complete series of observations" and had excluded only those that were interrupted by equipment problems, bad oil drop behavior, or atmospheric disturbances. The exclusions, in this reading, were scientifically justified rather than results-driven.

The Feynman Critique

The most famous commentary on the Millikan affair came from physicist Richard Feynman, in a 1974 commencement address at the California Institute of Technology that was later reprinted in his bestselling memoir, "Surely You're Joking, Mr. Feynman!"

Feynman pointed out something peculiar about the history of electron charge measurements after Millikan. If you plot them chronologically, each successive measurement is slightly higher than the last, slowly creeping up to the correct value over decades.

Why didn't they discover the new number was higher right away? It's a thing that scientists are ashamed of—this history—because it's apparent that people did things like this: When they got a number that was too high above Millikan's, they thought something must be wrong—and they would look for and find a reason why something might be wrong. When they got a number close to Millikan's value they didn't look so hard.

This is a perfect example of what Feynman called "cargo cult science"—work that has the superficial trappings of scientific rigor but lacks the crucial element of absolute intellectual honesty. Later experimenters weren't consciously falsifying their data. They were just unconsciously biased toward results that agreed with the established authority.

The culprit, it turned out, was Millikan's value for the viscosity of air. He had used a slightly incorrect number, which systematically biased all his charge measurements downward. Every subsequent experimenter who got a higher (and more correct) value was implicitly challenging Millikan's authority, so they looked harder for errors in their own work. Only over time, as the evidence became overwhelming, did the scientific consensus shift.

The Search for Smaller Charges

One of the most profound implications of Millikan's experiment was what it didn't find. Every charge was a whole-number multiple of the electron's charge. Nothing was ever measured as two-thirds or one-third of an electron.

This matters because modern particle physics predicts that quarks—the fundamental particles that make up protons and neutrons—carry charges of exactly one-third or two-thirds of an electron's charge. But quarks are never found in isolation; they're always bound together in combinations that add up to whole-number electron charges. A proton, for instance, contains two "up" quarks (each with charge +2/3) and one "down" quark (with charge -1/3), for a total charge of +1.

Could there be free quarks floating around, detectable by a modern version of the oil drop experiment?

From 1995 to 2007, researchers at SLAC, the Stanford Linear Accelerator Center, conducted a series of automated oil drop experiments specifically designed to search for fractionally charged particles. These experiments were far more sophisticated than Millikan's original setup, using computers to track and analyze over one hundred million individual droplets.

They found nothing. Not a single droplet with a fractional charge.

This null result is actually profound. It confirms that quarks really are confined inside composite particles and that we can't just knock them loose into the wild. The theory of quantum chromodynamics, which describes the strong nuclear force, predicts exactly this behavior—but it's always reassuring when nature cooperates with our equations.

Thomas Edison Changes His Mind

One footnote to this story involves the most famous inventor of the era. Thomas Edison had long believed that electric charge was a continuous quantity, not made up of discrete particles. This was the intuitive view—after all, when you turn a dimmer switch, the light seems to change smoothly, not in tiny jumps.

But after working with Millikan and Fletcher's apparatus firsthand, Edison became convinced. The evidence was simply too clear. When you watch oil droplets float and fall and see their charges always landing on exact multiples of the same base value, the quantization becomes undeniable.

Edison, for all his flaws, was an empiricist at heart. He followed the evidence.

A Rite of Passage

The oil drop experiment has since become a standard exercise in undergraduate physics laboratories around the world. Generations of students have squinted through microscopes at tiny droplets, calculated drag forces and electric fields, and derived their own measurements of the electron's charge.

It's a frustrating experiment to perform well. The droplets are maddeningly difficult to track. They drift. They evaporate. They pick up or lose charge unpredictably. Air currents disturb them. The measurements are finicky and the calculations are tedious.

But that's part of the point. Science at the frontier is messy, uncertain, full of judgment calls about which data to trust. The oil drop experiment teaches students not just a result but a process—the painstaking work of extracting signal from noise, of measuring the unmeasurably small through patient observation and clever experimental design.

And there's something profound about repeating, with your own eyes and hands, an experiment that revealed a fundamental constant of nature. The electron's charge isn't just a number in a textbook. It's something you can measure yourself, in a basement laboratory, with plates and oil and voltage and time.

The Elementary Charge Today

As of 2019, the elementary charge is no longer a measured quantity at all. In a major revision of the International System of Units, physicists decided to fix the electron's charge at exactly 1.602176634 times ten to the negative nineteenth coulombs, by definition.

This might seem like cheating, but it reflects a deep truth about metrology—the science of measurement. At some point, you have to anchor your system of units to something. The previous definition of the coulomb was based on the ampere, which was based on the force between current-carrying wires, which depended on various physical constants that could only be measured to finite precision.

Now, the electron's charge is one of seven fundamental constants that define the SI units. We've essentially said: whatever the charge of an electron is, that's our reference point. Everything else is measured relative to it.

It's a fitting tribute to Millikan and Fletcher's work. The quantity they struggled to measure, squinting at oil droplets for countless hours in a Chicago basement, has become so fundamental that we've built our entire system of physical units around it.

The electron's charge is no longer something we measure. It's something we count with.