What technique did Milikan use in his experiment?

In his famous oil drop experiment, Millikan used a microscope to observe tiny oil droplets that he had charged with static electricity. This allowed him to measure the charge of individual droplets as they were suspended between two charged plates. By adjusting the electric field between the plates and carefully measuring the balancing forces on the droplets, he could determine the charge on each droplet.

The use of a microscope was critical to Millikan's experiment as it enabled him to see the small droplets clearly and accurately assess their behavior under different electric forces. This precision was essential for his calculations to determine the fundamental charge of the electron, which became a significant contribution to our understanding of atomic structure and the nature of electric charge.