Millikan oil drop experiment. Cutaway artwork of the experiment carried out between 1909 and 1913 by US physicist Robert Andrews Millikan (1868-1953) to determine the charge on an electron. Oil is sprayed (upper right) into an upper chamber. The fine droplets pass through a pinhole into a lower chamber with a positive plate (red) and a negative plate (blue) with a high applied voltage. An X-ray source (lower left) produces X-rays (pink arrow) that creates a charge on the oil droplets (yellow) which are observed through a microscope eyepiece (lower right). Millikan found the charges always to be multiples of a single value, the charge on an electron,