PROBLEMS OF MODERN SCIENCE 



involve h. In fact, h is so fundamentally the basis 

 of the specification of the atom as a structure that 

 it must appear of necessity in the quantitative 

 expression of any phenomenon in which the atom 

 plays the part of a unit. In these considerations 

 taken alone there is clearly no difficulty. It is when 

 we compare such a specification, known from its 

 success in interpreting spectra, and for a variety 

 of other reasons, to be now practically inevitable, 

 with ordinary dynamics that the difficulties begin. 

 I can illustrate this with the simplest case of all 

 a hydrogen atom, whose form is now effectively 

 established from a long list of different lines of 

 evidence. 



This consists of an electron, with a charge ^, 

 rotating in a circle or other orbit, but considera- 

 tion of the circular orbit is sufficient of radius r, 

 round a relatively heavy positive charge, also of 

 magnitude e, at the centre of the circle. The 



e 2 

 attraction between them is , which supplies the 



r~ 



force necessary to keep the electron from flying 



off at a tangent. If the mass of the electron is m, 



and its angular velocity is co, 



'5 



9 



e 



mrco = -5- 



r 2 



which is, after all, only Kepler's law for the 

 planetary systems, co 2 oc 3 or the squares of 



3 2 



