2 Prof. R. A. Millikan on a new 



British physicists, to regard the value o£ e given above as 

 somewhat too high, a value being commonly adopted which 

 was about 2 per cent, lower. As this was much greater than 

 the necessary error in the method, I was anxious to see by 

 entirely new work whether a numerical error could have 

 crept into the former determination. 



Finally, the constant e has recently taken on added 

 importance, since not only does it now carry with it, as 

 formerly, the knowledge of the most important molecular, 

 atomic, and radioactive magnitudes (such as the exact 

 number of molecules in a given weight of any substance, 

 the absolute weight of any atom or molecule, etc.), but 

 all of the most significant of the radiation constants as 

 well (such as Planck's h, the Stefan-Boltzmann constant cr, 

 the Wien constant G 2 , all the X-ray constants, i. e., the 

 wave-lengths of characteristic X-rays) have recently been 

 found to depend for their most reliable evaluation * upon 

 the value of e. Further, if electricity exists in nature only 

 in exact multiples of e, then e is in a more complete sense 

 than any other physical quantity a natural unit, having none 

 of the arbitrariness about it which inheres in so-called 

 absolute units like the centimetre, the gram, and the second. 

 In a word, e is increasingly coming to be regarded both as 

 the most fundamental of physical or chemical constants, and 

 also as the one of most supreme importance for the solution 

 of the practical numerical problems of modern physics. It 

 seemed worth while, therefore, to drive the present method 

 for its evaluation — a method which is certainly exceedingly 

 exact if its validity is granted — to the utmost limits of its 

 possible precision. 



Accordingly, early in 1914, the work herewith reported 

 was begun. 



2. The Method. 

 For the sake of completeness, it may be stated again that 

 the method consists in capturing electrons f on an oil-drop 



* Physical Review, Millikan, vii. pp. 353-388 (1916), and Webster, 

 vii. p. 607 (1916). 



t The word electron is used with the meaning originally given to it 

 by Dr. Gr. Johnstone Stoney, viz., " the natural unit of electricity." 

 This use has been consistently followed by the most authoritative 

 writers, like Sir J. J. Thomson, Sir E. Rutherford, 0. W. Richardson, 

 N. Campbell, etc., all of whom speak in recent books or articles of 

 positive as well as negative electrons, though the mass associated with 

 the former is never less than that of the hydrogen atom. When an 

 electron is found associated with a mass but I¥ j^ of that of the 

 hydrogen atom it may be called " a free negative electron," or following 

 Sir J. J. Thomson " a corpuscle." 



