The Electronic Charge and the Oil Drop Method

Abstract

A study has been made of Millikan's oil drop method for determining the electronic charge. Owing to the uncertainty in the value of the viscosity of air, the value of the ratio e2/3/η23 has been determined and quoted rather than the electronic charge. Some of the factors that have been found to influence this determination are (1) the purity of the air; (2) the influence on the field of the hole in the top plate and the layer of oil which collects in the lower plate of the condenser; (3) corrections to Stokes's equation due to the influence of the walls of the apparatus; (4) oxidation of the oil; and (5) impurities in the oil. From a study of 47 drops of butyl sebacate, and using Stokes's analysis for the resistance to a sphere moving in a viscous medium, the value e2/3/η23 = (335.75±0.11)×10-5 (e e.s.u., η23 c.g.s.) has been obtained. This will be increased slightly if Oseen's modification to Stokes's analysis is applied, but the increase will be less than +0.04 per cent.