Velocity of the a particle or the Stopping Power. 515 

 Al next RaC. Au next RaC. 



1 339 



2 309 



3 305 



4 253 



5 246 



6 214 



Nowv/(339 x 305)/309 = l-04 ; 3-05/^/(309 x 253) = 1-09 

 ^(305 x 246)/253 = 1-08 ; 2-46/^(253 X 214) = 1-06 



The mean o£ these results is 1*07 . Thus the ionization 

 when the Au was uppermost is 7 per cent greater than when 

 it was next the RaC. 



Now, as already stated, when these two particular metal 

 sheets were inverted, there was a difference in the range of 

 the a particle after passing through them equal to "08 cm. 

 In this case the metals were at a distance of 1*5 cm. from 

 the radium. In order to calculate from this result a value of 

 the difference in the ionizations, for comparison with the 

 experimental value of 7 per cent., it is necessary to allow for 

 the fact that when the latter value was obtained the metals 

 were in contact with the radioactive surface, and, moreover, 

 many of the rays traversed the plates at a considerable incli- 

 nation to the normal. To do this it is necessary to make 

 some assumption as to the law connecting stopping-power 

 with speed. For this reason, and because the calculation is 

 rather lengthy, and the data not so reliable as might be 

 wished, I will only state here that I found the calculated 

 difference in the ionizations to be about 11 percent. There- 

 fore in this particular experiment the difference in range 

 when the plates were inverted w T as quite enough to account 

 for the observed difference in the ionizations. 



This may also be seen on general considerations. The 

 a particle that passed normally through the plates completed 

 afterwards a range of about 2*5 cm. in air. The difference 

 of range on inversion was at least *08 cm. Thus the difference 

 in the ionization caused by the normal rays was at least 

 •08/2*5 or 3 per cent. But many of the a particles crossed 

 the plates obliquely ; indeed, the relative importance of the 

 oblique to the normal radiation is a striking feature of t)ie 

 a rays. In the case of this important part of the radiation 

 the paths through the metals were longer, and therefore the 

 differences of range on inversion must have been greater. 

 On the other hand, the subsequent paths in the air were less ; 

 and therefore the differences of range were of still greater 



2N2 



