An object having a net charge of 18.2 μC is placed in a uniform electric field of 790 N/C directed vertically upward, i.e. in the direction opposite gravity. What is the mass, in grams, of this object if it "floats" in the field? (Assume g = 9.80 m/s2.)

Answer 1

Google Millikan oil drop experiment

for example:
http://ffden-2.phys.uaf.edu/212_fall2003.web.dir/Ryan_McAllister/Slide3.htm