232 ANNUAL KEPOET SMITHSONIAN INSTITUTION, 1910. 



free from all of these limitations, but it constitutes an entirely new 

 way of studying ionization and one whicli seems to be capable of 

 yielding important results in a considerable number of directions. 

 With its aid it has already been found possible^ — 



1. To catch upon a minute droplet of oil and to hold under observa- 

 tion for an indefinite length of time one single atmospheric ion or any 

 desired number of such ions between 1 and 150. 



2. To present direct and tangible demonstration, through the 

 study of the behavior in electrical and gravitational fields of this 

 oil drop, carrying its captured ions, of the correctness of the view 

 advanced many years ago and supported by evidence from many 

 sources that all electrical charges, however produced, are exact 

 multiples of one definite elementary electrical charge; or, in other 

 words, that an electrical charge, instead of being spread uniformly 

 over the charged surface has a definite granular structure, consist- 

 ing, in fact, of an exact number of specks or atoms of electricity, all 

 precisely alike, peppered over the surface of the charged body. 



3. To make an exact determination of the value of the elementary 

 electrical charge which is free from all questionable theoretical 

 assumptions and is limited in accuracy only by that attainable in 

 the measurement of the coefficient of viscosity of air. 



4. To observe directly the order of magnitude of the kinetic 

 energy of agitation of a molecule, and thus to bring forward new 

 direct and most convincing evidence of the correctness of the kinetic 

 theory of matter. 



5. To demonstrate that the great majorit}', if not all, of the ions of 

 ionized air, of both positive and negative sign, carry the elementary 

 electrical charge. 



6. To show that Stokes's law for the motion of a small sphere 

 through a resisting medium, breaks down as the diameter of the 

 sphere becomes comparable with the mean free path of the molecules 

 of the medium, and to determine the exact way in which it breaks 

 down. 



THE METHOD. 



The only essential modification in the method consists in replacing 

 the droplet of water or alcohol by one of oil, mercury, or some other 

 nonvolatile substance and in introducing it into the observing space 

 in a nev/ way. 



Figiu'e 1 shows the apparatus used in the following experiments. 

 By means of a commercial " atomizer " ^ A a cloud of fine droplets of 



1 The atomizer method of producing very minute but accurately spherical drops for the 

 purpose of studying their behavior in fluid media, was first conceived and successfully 

 carried out in January, 1908, at the Ryerson Laboratory, by Mr. J. Y. Lee, while he was 

 engaged in a quantitative investigation of Brownian movements. His spheres were blown 

 from Wood's metal, was. and other like substances which solidify at ordinary tempera- 

 tures. Since then the method has been almost continuously in use here, upon this and a 

 number of other nroblems. and elsewhere uDon similar nroblems. 



