The microscope sees a trajectory, not a charge
An illuminated oil drop moves between capacitor plates. The observer times its passage between marked positions, first under gravity and then with an electric field applied. The raw evidence consists of distances, times, applied voltages and environmental conditions. Charge is inferred through a mechanical model.
A stationary drop shows that electric force can balance effective weight. But its mass is not independently displayed by the microscope. The field-free terminal fall helps infer the drop size; density, air viscosity and buoyancy then enter the charge estimate. Calling the balance a direct reading of one electron hides these steps.
Read the evidence: R. A. Millikan (1913), On the Elementary Electrical Charge and the Avogadro Constant, Phys. Rev. 2, 109–143 — original scan
One useful balance, with its assumptions visible
For a spherical drop with terminal downward speed, the equation below balances buoyancy-corrected weight against viscous drag. The radius is positive, the two densities are oil and air, viscosity refers to air, and the slip correction is a dimensionless factor. The correction divides the continuum Stokes drag.
This is a low-Reynolds-number, terminal-motion approximation, not a law for an arbitrarily accelerating or evaporating droplet. If the electric force later points upward, its magnitude can be inferred from the opposing effective weight and drag. Its direction must be tracked before assigning a sign to the inferred charge.
Read the evidence: R. A. Millikan (1913), On the Elementary Electrical Charge and the Avogadro Constant, Phys. Rev. 2, 109–143 — original scan
Changing the same drop is a stronger comparison
A drop can change charge while remaining approximately the same mechanical object. Comparing its successive rise speeds helps isolate charge increments from the otherwise unknown drop mass. Millikan examined such sequences as well as many different drops; the 1913 paper summarizes complete series for 58 drops.
The proposed common unit must organize many inferred charges and increments within experimental uncertainty. One drop carrying an unknown number of electrons cannot by itself reveal that unit. Choosing an arbitrarily tiny divisor can always make approximate multiples easier to find, so resolution, scatter and the scale of observed increments matter.
Read the evidence: R. A. Millikan (1913), On the Elementary Electrical Charge and the Avogadro Constant, Phys. Rev. 2, 109–143 — original scanCaltechAUTHORS, Millikan’s 1913 charge paper — institutional record
The gas is part of the experiment
Millikan found that applying uncorrected Stokes drag produced a charge scale that depended on drop size. When a drop becomes small compared with molecular transport scales, air cannot be treated as a continuous fluid all the way to its surface. Pressure variation helped expose and estimate the correction.
Viscosity and temperature affect every inferred size and charge. Brownian motion adds fluctuations, while changing drop size or losing the tracked drop can invalidate a sequence. A narrow scatter among repeated readings does not rule out a shared calibration error. Agreement across sizes and pressures tests something different from timing the same motion more often.
Read the evidence: R. A. Millikan (1913), On the Elementary Electrical Charge and the Avogadro Constant, Phys. Rev. 2, 109–143 — original scan
Quantization and numerical accuracy are separate claims
The evidence supports a common charge unit in the drops and charge changes studied. It is not a measurement of electron shape, nor a universal search covering every possible carrier of electric charge. The mechanics used to infer the unit can remain classical while the resulting charge pattern is discrete.
In today’s SI the numerical value of the elementary charge is fixed by definition. That convention does not mean a droplet measurement has zero uncertainty. A modern experiment tests the realization of units and its apparatus; a 1913 determination estimated a constant under the conventions and calibrations then available.
Read the evidence: R. A. Millikan (1913), On the Elementary Electrical Charge and the Avogadro Constant, Phys. Rev. 2, 109–143 — original scanNIST, Defining the International System of Units — the 2019 ampere definition
Two Millikan measurements answer different questions
The oil-drop experiment establishes the charge scale needed to interpret the slope of photoelectric stopping potential against frequency. The photoelectric experiment then tests an energy–frequency relation. Neither experiment can be substituted for the other merely because both include a voltage.
There is no oil-drop acquisition Lab linked here. A plotted set of exact integer multiples generated by a simulator would illustrate the hypothesis, not establish it independently. A defensible reconstruction would retain timing records, corrections, rejected runs and uncertainties before testing whether one charge unit explains them.
Read the evidence: R. A. Millikan (1913), On the Elementary Electrical Charge and the Avogadro Constant, Phys. Rev. 2, 109–143 — original scanR. A. Millikan (1916), A Direct Photoelectric Determination of Planck’s h, Phys. Rev. 7, 355–388 — original scanRobert A. Millikan — official 1923 award facts