THEORY
MEASURING e
The electrical charge carried on an electron is a fundamental constant in Physics. The accepted value for e = 1.602 E-19 C. R.A. Millikan was the first to measure the constant. He did so in his oil drop experiment (1909-1913). Millikan also showed the discreteness of the charge, meaning only integral multiples of the charge exist.
In this experiment polystyrene spheres were used instead of oil drops. When the spheres are in the electric field three forces act upon them: gravitational, electric, and viscous. In measuring e the spheres are stationary so the viscous forces are equal to zero, and can be ignored in determining the charge on an electron. The charge can be measured using only the gravitational and electric forces.
With no electric field present, the only force on the sphere is the downward gravitational force (FG = mg). See picture A below. When the electric field is turned on then the two forces are acting on the sphere(FG & FE = qE). See picture B below. Assuming the plates are large enough that the electric field in the viewing area can be considered uniform, then FE = qV/d. When FG = FE then the particle will balance (on the vertical axis, motion on the horizontal axis cannot be stopped because of the way the sphere was injected into the chamber). See picture C below.
By assuming uniform mass of the sphere then q is related to the voltage by 1/V. From this you can see that a higher voltage will return a lower charge. A hig voltage is needed to see the elementary charge.