CH. VIIL] OF AN ELECTRON 87 



velocity was calculated by Sir George Stokes for 

 the case of a falling raindrop of radius r as follows : 



9 viscosity of air' 



where p is the excess density of the sphere over the 

 medium it moves in ; provided there is no finite 

 slip at the surface. The maximum possible effect 

 of surface slip which will occur to some extent 

 when the falling globules are very minute is to 

 make the possible terminal velocity half as great 

 again : in other words to convert the numerical 



2 1 



coefficient - into -. 



This simple formula gives the connexion between 

 the rate of fall of any small rain or fogdrop and its 

 size ; and by observation of this speed, therefore, 

 knowing the viscosity of air, it is possible to calculate 

 the dimensions of the falling drops. 



J. J. Thomsons Experiment of Counting. 



We have now all the materials ready for under- 

 ; standing the experiment to be performed,* so as to 

 count the ions which are produced in air under the 

 influence of Rontgen rays, or which have been produced 

 from a negatively electrified surface illuminated with 

 ultra-violet light. The apparatus for the former is 

 depicted in Fig. 12. The ionising beam of X-rays 

 enters the chamber A from above, through an earthed 

 aluminium lid which keeps it airtight. 



The rate of leak, which must be observed in order 

 to calculate N<m, is determined by connecting the 

 water and the aluminium plate to the terminals of 



* Phil. Mag., December, 1898, and December, 1899. 



