<Tr> <Td> </Td> <Td> This section needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (December 2010) (Learn how and when to remove this template message) </Td> </Tr> <P> Initially the oil drops are allowed to fall between the plates with the electric field turned off . They very quickly reach a terminal velocity because of friction with the air in the chamber . The field is then turned on and, if it is large enough, some of the drops (the charged ones) will start to rise . (This is because the upwards electric force F is greater for them than the downwards gravitational force F, in the same way bits of paper can be picked by a charged rubber rod). A likely looking drop is selected and kept in the middle of the field of view by alternately switching off the voltage until all the other drops have fallen . The experiment is then continued with this one drop . </P> <P> The drop is allowed to fall and its terminal velocity v in the absence of an electric field is calculated . The drag force acting on the drop can then be worked out using Stokes' law: </P> <Dl> <Dd> F d = 6 π r η v 1 (\ displaystyle F_ (d) = 6 \ pi r \ eta v_ (1) \,) </Dd> </Dl>

Why did the droplet of oil stay suspended in millikan's oil drop experiment