Example, lets say there is a negative drop of
oil with mass of m and charge of q between the
two uncharged plates. First the drop falls as
gravity acts on it and its drag is holding it
slightly airborne. When the plates are charged
with a electric difference (explained later, as
of now imagine that the top plate is positive,
and that the oil drop is attracted to it). Then
the oil drop moves upward with the force of qE
and gravity and its drag holds it down.
By turning the battery hooked up to the plates
on and off the oil drop can be pulled upward or
let fall or can be made to hang in the air. Thus
allowing Millikan to judge the strength of the
electric force acting on the particle.