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PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES Volume 3 APRIL 15. 1917 Number 4 A RE-DETERMINATION OF THE VALUE OF THE ELECTRON AND OF RELATED CONSTANTS By R. A. Millikan RYERSON PHYSICAL LABORATORY, UNIVERSITY OF CHICAGO Communicated, February 5, 1917 This re-determination of the most fundamental of physical constants was entered upon three years ago for three reasons. First, in 1913 results began to be published from Vienna,' which though obtained by a modification of my method,2 were wholly irreconcilable with those which I had found; and I accordingly wished to see whether I could find conditions under which the method failed. Second, there developed a tendency, especially among British physicists, to adopt a value of e about 2% lower than that which I had obtained, and as this difference was much greater than the necessary error in my method I was anxious to see, by entirely new work, whether a numerical error could have crept into the former determination. Third, the electron has recently taken on added importance because it has been found to carry with it not merely all molecular and atomic magnitudes, as heretofore, but also all of the most significant of the radiation constants, such as Planck's h, the Stefan-Boltzmann constant a, the Wien constant C2, all X-ray constants, i.e., the wave lengths of characteristic X-rays, etc.3 It seemed worth while therefore to drive my method, which is certainly exceedingly exact if its validity is granted, to the utmost limit of its possible precision. The method is the same as that used in the preceding determination,4 but the apparatus is new throughout and every constant entering into the value of e has been redetermined with increased care and precision. The condenser plates MN (fig. 1) consist of two optically flat brass surfaces 22 cm. in diameter, held apart by three small pieces of echelon p