606 Dr. H. Geiger and Mr. E. Marsden on the Laws o] 



original pencil, n the number of atoms in unit volume of the 

 materia], and t the thickness of the foil. The quantity 



2N*E 



= tt> 



raw 



where N<? is the central charge of the atom, and m, E, and u 

 are the respective mass, charge, and velocity of the a particle. 



The number of deflected a particles is thus proportional to 

 (1) cosee 4 (/>/2, (2) thickness of scattering material t if the 

 thickness is small, (3) the square of the central charge 1$e of 

 the atoms of the particular matter employed to scatter the 

 particles, (4) the inverse fourth power of the velocity u of 

 the incident a, particles. 



At the suggestion of Prof. Rutherford, we have carried 

 out experiments to test the main conclusions of the above 

 theory. The following points were investigated : — 



(1) Variation with angle. 



(2) Variation with thickness of scattering material. 



(3) Variation with atomic weight of scattering material. 



(4) Variation with velocity of incident a particles. 



(5) The fraction of particles scattered through a definite 



angle. 



The main difficulty of the experiments has arisen from the 

 necessity of using a very intense and narrow source of 

 a particles owing to the smallness of the scattering effect. 

 All the measurements have been carried out by observing 

 the scintillations due to the scattered a. particles on a zinc- 

 sulphide screen, and during the course of the experiments- 

 over 100,000 scintillations have been counted. It may be 

 mentioned in anticipation that all the results of our investi- 

 gation are in good agreement with the theoretical deductions 

 of Prof. Rutherford, and afford strong evidence of the cor- 

 rectness of the underlying assumption that an atom contains 

 a strong charge at the centre of dimensions, small compared 

 with the diameter of the atom. 



(1) Variation of Scattering with Angle. 



We have already pointed out that to obtain measurable 

 effects an intense pencil of: a particles is required. It is 

 further necessary that the path of the a particles should be 

 in an evacuated chamber to avoid complications due to the 

 absorption and scattering of the air. The apparatus used is 

 shown in fig. 1, and mainly consisted of a strong cylindrical 

 metal box B, which contained the source of a. particles R. 



