Editing Oil drop experiment (section)

===Apparatus===
{{Refimprove section|date=December 2010}}

[[Image:Simplified scheme of Millikan’s oil-drop experiment.svg|right|thumb|Simplified scheme of Millikan's oil drop experiment|576x576px]]
[[Image:Millikan’s oil-drop apparatus 1.jpg|right|thumb|Oil drop experiment apparatus|335x335px]]
Millikan's and Fletcher's apparatus incorporated a parallel pair of horizontal metal plates.  By applying a potential difference across the plates, a uniform electric field was created in the space between them. A ring of insulating material was used to hold the plates apart. Four holes were cut into the ring, three for illumination by a bright light, and another to allow viewing through a microscope.

A fine mist of oil droplets was sprayed into a chamber above the plates.  The oil was of a type usually used in [[vacuum]] apparatus and was chosen because it had an extremely low [[vapour pressure]]. Ordinary oils would evaporate under the heat of the light source causing the mass of the oil drop to change over the course of the experiment. Some oil drops became electrically charged through friction with the nozzle as they were sprayed.<ref name=Pekola-2013/>{{rp|p=1423|q=In those experiments, the oil droplets were, however, charged randomly by an uncontrollable process of absorption of ions which exist normally in air.}}

Alternatively, charging could be brought about by including an ionizing radiation source (such as an [[X-ray tube]]). The radiation source can remove electrons from the droplet, as well as the air surrounding it. Some of the droplets get negatively charged by receiving a net excess of electrons that were ionized out of the air molecules around it. Positively charged drops will be pulled downwards both by gravity and the electric field, and will not reach mechanical equilibrium. The negatively charged droplets are the important ones for the experiment, because they can reach mechanical equilibrium.{{cn|date=March 2025}}

The droplets entered the space between the plates and, because they were charged, could be made to rise and fall by changing the voltage across the plates.