In Robert Millikan’s oil-drop experiment, what fundamental electrical quantity did he measure?

Robert Millikan’s oil-drop experiment measured the elementary electric charge.

Between 1909 and 1911, Millikan and Harvey Fletcher suspended tiny oil droplets between two horizontal metal plates. By adjusting the electric field, they balanced the downward force of gravity against the electrical force on individual charged droplets. Measuring how the droplets moved when the field changed allowed the researchers to calculate their charges.

The measured charges were always whole-number multiples of a smallest value, about 1.6 × 10−19 coulombs. This supported the idea that electric charge is quantized rather than continuously divisible. The smallest value was identified with the charge of a single electron.

The experiment is sometimes described as measuring the electron itself, but its main result was the value of elementary charge. Millikan later received the 1923 Nobel Prize in Physics, while J. J. Thomson’s earlier cathode-ray work had established the electron’s charge-to-mass ratio.