Millikan’s Oil Drop Apparatus is a scientific device designed by American physicist Robert Millikan in 1909 to determine the charge of an electron. The apparatus consists of a glass chamber filled with air or gas, a microscope, and a source of charged oil droplets.
The procedure involves introducing charged oil droplets into the chamber and observing their motion through the microscope. By measuring the rate of fall or rise of the droplets under the influence of gravity and an applied electric field, Millikan was able to determine the magnitude of the charge on each droplet.
By repeating the experiment with different droplets and electric fields, Millikan was able to determine that the charge on each droplet was a multiple of a fundamental unit of electric charge, which he identified as the charge of a single electron. This experiment was instrumental in accurately determining the value of the charge of an electron and contributed significantly to the development of modern atomic theory.