Oil Drop Method
Millikan's requirements in designing his elegant and ingenious oil drop method were:
(1) the creation of the smallest possible, completely spherical, homogenous body. This body must have a constant mass in the absence of interfering gravitational force and convection currents. And
The apparatus Millikan built admitted an oil drop (from an atomizer) into a stable atmosphere of fixed length between two metal plates which applied a fixed electric field to the drop. A switch in the circuit allowed the field to be turned on and off.
Experimentation with the apparatus involved varying the parameters: the measurable distance the drop traveled, the size of the plates and the methods used to change the sphere's electrical charge.
Finally the size of the oil drop was a thousandth of a millimeter in diameter, the centimeter of travel path for the oil drop was obtained from 22 cm diameter plates positioned 16 mm apart and the strength of the applied charge was about 6,000 volts per cm. Each of these values proved optimum; variations would ruin the experiment.