Who determined the absolute value of the charge on an electron?

The absolute value of the electron’s charge was first precisely measured by Robert A. Millikan in 1909 through his landmark oil-drop experiment. Before this, J. J. Thomson (1897) had discovered the electron and determined its charge-to-mass ratio (e/m), but the exact charge remained unknown. Millikan’s work provided the missing piece by independently determining the electron’s charge, allowing scientists to calculate its mass with accuracy.

In the oil-drop experiment, Millikan sprayed tiny oil droplets into a chamber where they were allowed to fall under the influence of gravity. These droplets were then exposed to X-rays, which ionized the air molecules, transferring electrons to the oil droplets and giving them a small electric charge. By applying an electric field across two metal plates, Millikan could balance the gravitational force pulling the droplet downward with the electric force pushing it upward.

By carefully adjusting the voltage, he caused individual droplets to suspend motionless in mid-air. Through precise measurements of the electric field strength and droplet size, Millikan calculated the charge on each droplet. He found that the measured charges were always whole-number multiples of a smallest, fundamental charge — that of a single electron.

The value he determined was approximately 1.602 × 10⁻¹⁹ coulomb, which is still accepted today with minor refinements. This discovery was crucial in validating the concept of quantized electric charge and establishing the discrete nature of subatomic particles.

Millikan’s experiment also strengthened the understanding of atomic structure and provided experimental support for the quantum theory emerging at that time. His meticulous approach earned him the Nobel Prize in Physics in 1923, solidifying his place among the pioneers of modern physics.

Through this experiment, the fundamental electrical unit of matter—the electron—was characterized precisely, marking one of the most important milestones in the field of experimental physics.