744 Mr. J. Chadwick on the Charge on the 



§ 5. The Laic of Force. 



In his theory of the single scattering of a particles 

 Rutherford assumed that the law of force between the 

 u particle and the nucleus was that of the inverse square. 

 It has been pointed out by Darwin * that a direct test of 

 the law of force is given by the dependence of scattering 

 on the velocity of the u particle. If the force around the 

 nucleus vary with the distance as 1/rP, then the number of 

 scattered a. particles, other conditions remaining constant, 



is proportional to I — g I . The experiments of Geiger and 



Marsden showed that the number of scattered particles 

 depended on the fourth power of the velocity to the 

 nearest integral power. Combined with the observed law 

 of angular distribution of the scattered particles, this leaves 

 no doubt as to the general validity of the inverse square law. 

 In view, however, of the accuracy attained in the measure- 

 ment of the nucleus charge, in which the inverse square law 

 is assumed, it was considered necessary to repeat this test in 

 a stricter manner. 



The same apparatus was used, but the mica sheet at 

 was removed and the ZnS screen waxed on in its place. 

 This was necessary in order that short range ex. particles 

 could reach the screen. By means of a ground-glass 

 joint H mica sheets were brought in front of the source 

 to cut down the velocity of the a particles incident on the 

 scattering foil. The stopping powers of the mica sheets 

 were measured by adjusting the pressure of air in the box 

 until the direct pencil of a rays just failed to reach the 

 screen. The velocity was calculated from the emergent 

 range, using Geiger' s relation u z = aR. A slight correction 

 of the measured ranges was necessary, since the a particles 

 which hit the scattering foil travel through the mica at a 

 small angle. 



The scattering foil was platinum weighing 1*58 mgm. 

 per sq. cm. The number n of scattered particles was 

 counted for three velocities, about 1600 particles being 

 counted for each. 



The results are given in the table : 



Mica sheet. Relative w 4 . nu^. 



1 100 



1 -549 101 



2 -232 103 



* Darwin, Joe, cit. 



