The Millikan Oil-Drop Experiment
Electric charge is discrete
First published: R. A. Millikan, "On the Elementary Electrical Charge and the Avogadro Constant", *Physical Review* 2 (1913): 109–143.
Charged oil droplets suspended in an electric field carry charges that come only in integer multiples of *e*.
Tiny oil droplets, sprayed into a chamber and ionised by X-rays, are observed through a microscope as they fall between two horizontal plates. By adjusting the voltage to suspend a droplet against gravity, the charge on it can be computed. Millikan and Fletcher observed that the charges always came in integer multiples of a single elementary charge — *e* ≈ 1.6 × 10⁻¹⁹ coulombs. Continuous distributions of charge were ruled out. The result is the cleanest demonstration of the discreteness of electric charge — and thus a direct empirical anchor for atomism, against the continuum models still defended by Mach and Ostwald at the time. Millikan was awarded the Nobel Prize in 1923. The experiment has a complicated historiography (Millikan was selective about which data to report) but the basic discreteness result has been confirmed thousands of times since.
Formulation
Oil-drop atomiser → ionising radiation → parallel-plate capacitor with adjustable voltage. Observe drop velocity under gravity (no field) and at equilibrium (with field). Compute charge from Stokes' law and force balance. Predicted (atomist): discrete spectrum, q = ne. Observed: all measured charges fit q = ne with e ≈ 1.59 × 10⁻¹⁹ C (modern: 1.602).
Dimensions Engaged
Matter
A clean demonstration that Matter · Conservation is discrete at the relevant scale: electric charge — and by extension, electrons — exists in indivisible units.
Energy
Indirectly bears on Energy · Conservation by anchoring the elementary charge that enters into all electromagnetic-energy quantities.
Responses — How Schools Engage
Affirms / takes the bait 5
A canonical empirical decision: continuous-charge theories are simply ruled out by the data. Atomism about charge becomes scientific orthodoxy.
Scientific realism: electrons are real, discrete, carrying a definite charge. The experiment measures a property of the world, not a calculational convenience.
Charge as a discrete structural quantity, with *e* a fundamental constant of nature. The experiment fixes a node in the structural inventory of the physical world.
Operationally exemplary: a quantitative measurement with a clear theoretical commitment, replicable across laboratories, fixing a constant of nature.
The decision between continuum and atomistic electrodynamics is settled by direct observation, not by theoretical preference. A model case for how physics should be done.
Denies / rejects the premise 1
The "elementary charge" is a theoretical posit that reduces, on close inspection, to a pattern in observations. Phenomenalists accept the data and demur on the realist gloss.
Related Experiments
Experiments engaged by an overlapping set of schools — likely to surface the same fault lines.
Further reading
- Holton, "Subelectrons, Presuppositions, and the Millikan-Ehrenhaft Dispute", *Hist. Stud. Phys. Sci.* 9 (1978)
- Franklin, *The Neglect of Experiment* (1986), ch. 6
- Goodstein, *Out of Gas* (1990) — on scientific replication
Related Historical Debates
Debates that share dimensions and/or aligned schools with this experiment.
Personas Most Aligned With This Experiment
Ranked by total declared-influence weight in the schools that respond to this experiment.
Works Most Aligned With This Experiment
Ranked by total declared-influence weight in the schools that respond to this experiment.
Related Contemporary Dilemmas
Dilemmas that engage the same dimensions as this experiment.