Millikan's measurement of the electron charge

As the oil drops left the spray they became negatively charged and when a potential difference was placed across the plates it was possible to adjust the size of this potential so as to give an electrostatic force (Eq) that just balanced the gravitational pull on the drop (mg).

mg = Eq = EV/d

where E is the field between the plates (= V/d where V is the potential difference between them and d their separation.)

Millikan found that all the charges were multiples of one basic charge namely 1.6x10^-19 C and he reasoned that since he found no charge of smaller value this must be the charge on the electron.