210 ANNUAL KEPOIIT SMITHSONIAN INSTITUTION, 1937 



by J. J. Thomson and his colleagues, Townsend, H. A. Wilson, and 

 C. T. R. Wilson. I think a brief r6siim6 of attempts to measure the 

 electron's charge will throw an interesting sidelight on the versatility 

 of scientific attack on a difficult problem. 



The first attempts were by Townsend, by measurements on the 

 motion and electrification of fog produced when electrolytic gas was 

 bubbled into a region of air which was slightly supersaturated with 

 water vapor, but too many uncertainties were involved to make this 

 work convincing. The first accepted results were by J. J. Thomson, 

 who, after an earlier attempt, employed a technique of producing fog 

 under controlled conditions, developed by his colleague, C. T. R. 

 Wilson, whoso method was refined further by his pupil, H. A. Wilson. 



It had long been known that water droplets of fog do not form in 

 air which is somewhat supersaturated with water vapor unless there 

 are nuclei, like specks of dust, on which the moisture can condense. 

 Later, Townsend found that fog will also condense on ions, and more 

 readily on negative than on positive ions. C. T. R. Wilson designed 

 an apparatus in which dust-free air could be supersaturated with mois- 

 ture sufficiently to permit condensation of fog droplets on negative 

 but not on positive ions, which were produced by some convenient 

 ionizing agent. So a fog was formed, in which each droplet of water 

 was condensed on a negative ion. Thomson employed this apparatus 

 in the following manner. 



Of course, this fog gradually settled downward under the pull of 

 gravity — slowly because the drops were small compared with the 

 viscous resistance of the air through which they fell. It was like the 

 slow settling of dust on the furniture and floor of a room. But the 

 theory of the rate at which spheres move when a force drives them 

 through a viscous medium was already well kno^vn, owing to Stokes's 

 law. From this law, measurement of the rate of fall of the fog in 

 centimeters per second as measured by a little telescope focused on the 

 top edge of the fog, combined with knowledge of the force of gravity 

 and the viscosity of air, enabled Thomson to calculate the size of the 

 individual fog droplets. Dividing the total amount of water in the 

 fog by the amount in one drop gave him the total number of fog drop- 

 lets, and therefore the total number of negative ions. H. A. Wilson 

 added the refinement of superposing an electric field on the gravita- 

 tional field which pulled the drops through the air. Then, as the fog 

 settled to the bottom of the apparatus, it deposited its electric charge, 

 which altogether, was large enough to be measured with an electrom- 

 eter. So, dividing this total charge by the number of ions composing 

 it gave, as the charge of one ion, 3.4X10"'° electrostatic units. This 

 was the first real measurement of the charge of an electron, and was 

 the value quoted in the tables of phj'^sical constants when I became a 

 graduate student in 1910. 



