564 Sir E. Rutherford on Collision of 



deflexion of the pencil of a rays from radium C. Since the 



Mr 

 value of ^ for a particles from radium has been accurately 



determined in other experiments and found to be 3*98 x 10 5 *, 



the average value of for the pencil of H atoms is 



2 - 74x 10 5 . Now, on passing the slit A, the maximum range 

 of the H atom is 28 — 3'-l = 24() cm., while the minimum 

 range for H atoms to be observed on the screen is 

 28 — 14'4 = 13'6 cm. Since the range of the H atom is 

 proportional to the cube of its velocity, the velocity of the 

 H atoms observed which passed through the magnetic field 

 lies between "96i/o and '79i« , where w is the maximum velocity 

 of the H atom due to an « particle from radium C. Since the 

 relation between number and velocity was approximately 

 linear over this range, the average velocity of the H atoms 



was '87m . Consequently the value of - ° for the swiftest 



H rays produced by a particles from radium C is 

 1/-87x2'74x10 5 = 3'15x10 5 . On the collision theory, the 

 velocity u of the H atoms of mass m is given by 



2M 



U = Trjf . V COS C7, 



and the maximum value 



u Q = l'6r . 



The value of — - for H atoms carrying unit charge 



should consequently be 3"2x 10 5 . The agreement between 

 theory and experiment is closer than we should expect 

 considering the difficulty of the measurements. In these 

 experiments it was found that all the H atoms carried a 

 positive charge, and no sign of scintillations was observed 

 indicating the presence of negatively charged or neutral 

 atoms. 



Electrostatic deflexion of H atoms. 



The determination of the deflexion of H atoms in an 

 electric field was a much more difficult and lengthy process. 

 The experimental arrangement finally adopted is shown in 



* Rutherford and Robinson, Phil. Mag. xxviii. p. 552 (191-4). 



