436 



SCIENCE 



[N. S. Vol. XXXII. No. 822 



SPECIAL ARTICLES 

 THE ISOLATION OF AN ION, A PRECISION MEASURE- 

 MENT OF ITS CHARGE, AND THE CORRECTION 

 OF STOKES's LAW^ 



§ 1. Introduction. — There is presented here- 

 with a new method of studying gaseous ioni- 

 zation, with the aid of which it has been found 

 possible : 



1. To catch upon a minute droplet of oil 

 and to hold under observation for an indefi- 

 nite length of time one single atmospheric ion 

 or any desired number of such ions between 1 

 and 150. 



2. To present direct and tangible demon- 

 stration, through the study of the behavior 

 in electrical and gravitational fields of this oil 

 drop carrying its captured ions, of the cor- 

 rectness 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; in other words, that an 

 electrical charge, instead of being spread uni- 

 formly over a charged surface, has a definite 

 granular structure, consisting, 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 this elementary electrical charge, 

 which is free from all questionable theoret- 

 ical assumptions and is limited in accuracy 

 only by the accuracy which is attainable in 

 the measurement of the coefficient of viscosity 

 of air. 



4. To observe directly the order of magni- 

 tude 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 majority 

 of the ions of the air of both positive and 

 negative sign, carry the elementary electrical 

 charge, and to present convincing evidence 

 that some atmospheric ions carry exact mul- 

 tiples of this charge; in other words, that the 



'At the request of the editor this abridgment 

 of a paper presented on April 23, 1910, before the 

 American Physical Society is published in Science. 



phenomena of valency are exhibited to some 

 extent in gaseous ionization. 



6. To show that the law of motion of a 

 small sphere through a resisting medium, 

 commonly known as Stokes's law, 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 investigation by means of which these 

 results have been obtained differs from most 

 of the equally important ones which are car- 

 ried on in the physical laboratory, in that the 

 method used is so simple, and the conclusions 

 follow so inevitably from the experimental 

 data that even the man on the streets can 

 scarcely fail to understand the method or to 

 appreciate the results. 



§ 2. The Method.— The method by which 

 these results have been obtained and by which 

 still further important results bid fair to be 

 obtained grew out of some experiments which 

 were presented in a preceding paper.° It is in 

 brief as follows : A cloud of fine droplets of 

 oil, or of mercury, or of some other non-vola- 

 tile substance is blown by means of an ato- 

 mizer' over a horizontal air condenser and a 

 few of the droplets in this cloud are allowed 

 to fall through a pinhole in the middle of the 

 upper plate of this condenser into the space 

 between the plates. The pinhole is then 

 closed for the sake of shutting out air cur- 

 rents. The condenser used consists in most 

 of the experiments of two heavy, circular, 

 and accurately planed brass plates, 20 cm. in 

 diameter, held exactly 16 mm. apart by means 

 of three small ebonite posts. The plates are 



■Phil. Mag., 19, p. 209, 1910. 



' 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 investiga- 

 tion 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 problems, and elsewhere upon similar 

 problems. 



