﻿140 Prof. E. Rutherford on the Retardation of the 



Curve I. The velocity of the rays from the bare wire is taken 

 as 100. The abscissa? represent the number of layers of foil 

 over the active wire. Curve II. shows the energy curve, the 

 energy of the a particle escaping from the bare wire being- 

 taken as 100. This was obtained by taking the squares of 

 the observed velocities. It will be seen that the points lie 

 nearly on a straight line, which cuts the axis of abscissae at a 

 distance corresponding to 16*9 layers of foil. Each layer of 

 foil absorbs absorbs about 6*0 per cent, of the maximum 

 energy of the a particle. 



The photographic effect of the rays after passing through 

 14 layers of foil was not more than 5 per cent, of that pro- 

 duced by the bare wire in the same interval. The actual 

 energy of the a particle at this stage is '18 of the initial 

 energy, so that it is seen that the photographic intensity of 

 the a particles near the end of their course falls off more 

 rapidly than their kinetic energy. 



Taking as the most probable value that the a rays are com- 

 pletely absorbed in 14*4 layers of foil, which would correspond 

 to 7*06 cms. of air at atmospheric pressure and temperature, 

 we see that each layer of foil on an average corresponds to 

 slightly less than 0'50 cm. of air. 



Now the continuation of the energy curve cuts the axis 

 for a thickness of aluminium corresponding to 16*9 layers 

 of foil, which is equivalent to a distance of air equal to 

 (16'9 — 14*4) x 0*50, or 1*25 cms. of air beyond the distance 

 7*06 cms. required for complete absorption. Consequently, 

 the velocity of the a particles which have a range r cms. in 

 air after passing through an absorbing screen is proportional 

 to \/V + 1*25. The ratio K of the velocity V of the « particle 

 of range r to the velocity V of the a. particle from the bare 

 wire is given by 



K= T -= ^ +1 ' 25 = ^48xA+T25. 

 r o x/7-06 + 1'25 



For example, the range r of the a. particles from radium C 

 after passing through 8 layers of foil (equal to 4*0 cms. of 

 air) is 3*06 cms. The ratio K is consequently '12. 



We therefore see that the velocity of the a particle of any 

 known range in air can be deduced from a simple formula. 



We shall show in a later paper that the value of ejm is the 

 same for the a particles from radium A, radium C, and 

 radium F (polonium). It thus appears probable that the 

 a particles from the various radium products have all the same 

 value of ejm. If this be the case, we are at once able to 

 deduce from the above formula the initial velocitv of the 



