“The electric charge carried by a particle may be calculated by measuring the force experienced by the particle in an electric field of known strength. Although it is relatively easy to produce a known electric field, the force exerted by such a field on a particle carrying only one or several excess electrons is very small. For example, a field of 1000 volts per cm would exert a force of only 1.6 l0^-9 dyne on a particle bearing one excess electron. This is a force comparable to the gravitational force on a particle with a mass of l0^-l2 (one million millionth) gram.” (PASCO, Manual Nº 012-06123C, p. 1).