312 THE REALITIES OF MODERN SCIENCE 



was smaller. He therefore doubted the applicability 

 of Stokes's Law to spheres of such a small size. One 

 of his students, Arnold, tested Stokes's Law for a wide 

 range of diameters by using small spheres formed of 

 rose-metal, a substance with a low melting point. 

 The velocity was found to be correctly expressed by 

 Stokes's Law only when the radius r of the sphere was 

 large as compared to the mean free path of the mole- 

 cules of the air. The mean free path being inversely 

 as the pressure of the air the true velocity was found 

 to be expressed as 



pr, 



where p is the pressure, A is a constant, and Vi is 

 the velocity of fall as calculated by Stokes's Law. 



Millikan therefore determined the value of e by 

 using droplets in air under various pressures, and thus 

 found not only the value of the correction factor A 

 but also the true value of e. As a further check he 

 repeated his experiments with drops so large that 

 Stokes's Law would hold for fall through air at atmos- 

 pheric pressure. He then obtained e by substituting 

 directly in equation (4). 



The value of e as obtained by this method is 

 4.774X10- 10 electrostatic unit or 1.591X10- 20 electro- 

 magnetic unit. This determines Avogadro's constant, 

 N, as 6.065X10 3 . From this the mass of a hydrogen 

 atom is obtained as 1.662X10" 24 gram. Using the 

 best value of e/m for the electron and the above value 

 of e gives the mass of the electron as 1/1845 part of the 

 hydrogen atom, or 9.01 X10~ 28 gram. Upon the as- 

 sumption that the entire mass is electromagnetic and 



