Robert Millikan, at the University of Chicago, designs between 1909 and 1913 the oil-drop experiment: he suspends tiny electrically charged oil drops between two metal plates, balancing the force of gravity with an adjustable electric field until the drop remains motionless. Repeating the experiment with thousands of drops, Millikan determines that the electric charge of any object is always an integer multiple of a fixed minimum value — the electron's charge — experimentally demonstrating that electricity is quantized in discrete units, not a continuous fluid. Between 1914 and 1916, Millikan sets out to test Einstein's 1905 prediction about the photoelectric effect, which he initially considered unlikely; after years of extremely precise measurements, Millikan confirms every quantitative aspect of Einstein's equation, including an independent and very precise determination of Planck's constant from purely experimental data. The historical irony is notable: Millikan, a skeptic of the quantum theory of light, ends up providing the decisive experimental evidence that confirms it.