680 Prof. E. Rutherford on the 



§ 6. Comparison of Theory with Experiments. 



On the present theory, the value of the central charge K^ 

 is an important constant, and it is desirable to determine its 

 value for different atoms. This can be most simply done by 

 determining the small fraction of a or ft particles of known 

 velocity falling on a thin metal screen, which are scattered 

 between cf> and cf>-{-dcf) where </> is the angle of deflexion. 

 The influence of compound scattering should be small when 

 this fraction is small. 



Experiments in these directions are in progress, but it is 

 desirable at this stage to discuss in the light of the present 

 theory the data already published on scattering of a and /3 

 particles. 



The following points will be discussed ; — ■ 



(a) The " diffuse reflexion " of a particles, i. e. the 

 scattering of a particles through large angles (Geiger 

 and Marsden). 



(b) The variation of diffuse reflexion with atomic weight 

 of the radiator (Geiger and Marsden). 



(c) The average scattering of a pencil of a rays trans- 

 mitted through a thin metal plate (Geiger). 



(d) The experiments of Growth er on the scattering of 

 fi rays of different velocities by various metals. 



(a) In the paper of Geiger and Marsden (loc. cit.) on the 

 diffuse reflexion of a. particles falling on various substances 

 it was shown that about 1/8000 of the ex. particles from radium 

 G falling on a thick plate of platinum are scattered back in 

 the direction of the incidence. This fraction is deduced on 

 the assumption that the ex. particles are uniformly scattered 

 in all directions, the observations being made for a deflexion 

 of about 90°. The form of experiment is not very suited for 

 accurate calculation, but from the data available it can be 

 shown that the scattering observed is about that to be expected 

 on the theory if the atom of platinum has a central charge of 

 about 100 e. 



(/;) In their experiments on this subject, Geiger and 

 Marsden gave the relative number of a particles diffusely 

 reflected from thick layers of different metals, under similar 

 conditions. The numbers obtained by them are given in the 

 table below, where z represents the relative number of 

 scattered particles, measured by the number of scintillations 

 per minute on a zinc sulphide screen. 



