Millikan's apparatus for measuring the charge on the electron, c 1915.

Diagram, fig 8, from 'The electron: its isolation and measurement and the determination of some of its properties' by Robert Andrews Millikan, (Chicago, 1917), shows the apparatus used by Millikan to measure the charge on the electron. He did this by observing the motion of tiny charged drops of oil in an electrified field between two metal plates at the bottom centre of the diagram. Millikan (1868-1953) was awarded the Nobel Prize for Physics in 1923 for accurately determining the charge on the electron, e, which is the smallest quantity of charge which can exist.