The essential idea is that, by comparing the magnitude of the electrostatic force, qE, exerted by an electric field E on a charge q with that of the gravitational force, mg, exerted by gravity on the mass bearing the charge, one may determine q, the quantities E, m, and g, all being known or easily measurable. When this is done, it turns out that the value of q (at least for very small oil- drops) always turns out to be a fairly small multiple of a certain value, which is thus assumed to be the basic unit in which charges come.