Millikan's apparatus for the determination of the elementary charge

From 1909 until 1911 Robert A. Millikan together with his assistant Harvey Fletcher conducted experiments to determine the elementary charge e, Avogadro's number, the Faraday constant F and properties of the Brownian motion. They used a self- made oil droplet apparatus and published their results in several articles.

The core element of Millikan's setup are two horizontally oriented condensator plates. Aerolised oil particles enter into the condensator through a minute hole in the upper plate. When the plates are charged, the mostly ionised oil droplets are affected by gravitational pull and the electrostatic field. The droplets are considered to have reached their final velocity when entering the condensator area. Since each droplet has an individual size and an individual nummber of charges, the combined forces have a unique effect on every single one.

Hence some droplets are falling whilst others can rise between the condensator plates. A very distinct few may also hover in the condensator. The oil droplets observed through a telescope and illuminated laterally with a bright light source. A scale reading and a stopwatch are used to calculate the rate of ascent and descent of the droplets.