Atomic Nucleus and the Lair oj Force. 737 



but this could be explained by the high voltages which would 

 be necessary to stimulate its emission. The penetrating 

 7 rays emitted in some radioactive transformations are 

 known to be of far higher frequency than corresponds 

 to the K series, and their origin is not yet clear, though 

 it lias been suggested by Rutherford that they come from 

 the nucleus itself. 



It appears, therefore, of great importance to measure the 

 unclear charge as accurately as possible by a direct method, 

 and to compare the value obtained with the atomic number. 

 Accordingly the experiments to be described in this paper 

 were undertaken. 



§ 2. Method oj Measurement of Nuclear Charge. 



The only direct method we have of measuring the charge 

 on the nucleus of an atom is from the single scattering of 

 * particles. Sir Ernest Rutherford has shown that, if a 

 pencil of a. particles falls on a thin sheet of matter, the 

 number of scattered a particles which fall per second on 

 unit area of a screen, distant r cms. from the point of 

 incidence of the pencil in a direction making an angle <£ 

 with it. is given by 



Q»^ 2 cosec 4 <ft/2 

 ~16V ' 



where Q = number of a particles per second in the original 

 pencil, 

 n = number of atoms in unit volume of the foil, 

 t = thickness of the foil, 



and h = — s. Ne, 

 mir 



where N« is the nuclear charge of the scattering atom 

 and m, u, E are the mass, velocity, and charge of the 

 a particle. 



In this calculation it was assumed that the collision of 

 the a particle with the nucleus does not set the atom in 

 motion. Darwin * modified the calculation to include this 

 effect, and found that if M be the mass of the nucleus and 

 greater than m, cosec 4 0/2 in the above expression must be 

 replaced by 



co S ec^/2- 2 (^ + (l-| a in^)g)V.... 

 * Darwin, Phil. Mag-, xxvii. p. 499 (1914). 



