This Demonstration simulates the famous Millikan oil-drop experiment that provided evidence that electric charge is quantized, occurring only as an integer multiple of an elementary charge, now known to equal (coulombs). In a chamber, an atomizer emits oil drops that fall through a hole in the upper plate and are ionized by X-rays before hitting the lower plate. A high voltage between the two plates is adjusted so that the oil drops are suspended, and they are viewed through a microscope. Millikan won the Nobel Prize in Physics for this experiment in 1923.