170 Prof. E. Rutherford on some 



Decrease of Velocity of the ex. particle in passing 

 through matter. 



Some experiments were made to determine the relative 

 velocity of the a particles from radium C after passing through 

 known thicknesses of aluminium. The apparatus shown in 

 fig. 1 was employed, and successive layers of aluminium-foil 

 of thickness "00031 cm. were placed over the active wire. 

 The amount of deviation of the rays is inversely proportional 

 to their velocity after their passage through the screen. The 

 impressions obtained on the plate were all clear and distinct, 

 and the breadth of the band was very nearly the same in all 

 cases. This shows that the rays after traversing a metal 

 screen were still homogeneous, although their velocity had 

 been reduced. 



A clear photographic impression was obtained with twelve 

 layers of foil over the wire, but it was not found possible to 

 obtain any effect through 13 layers. This result shows that 

 the photographic action, like the ionizing action of the a rays, 

 ceases very abruptly. The photographic effect of the a rays 

 ceases after they have passed through a thickness of aluminium- 

 foil greater than '0037 cm. and less than '0040 cm. 



The results obtained are shown in the following table. 

 Assuming that the value of ejm is constant, the third column 

 gives the velocity of the a particles after passing through the 

 aluminium. This is expressed in terms of V , the velocity 

 of the ol particles when the screens are removed. 



Number of layers 



Distance 2d 



Velocity of 



of aluminium-foil. 



between bands. 



a, particle. 







1*46 mms. 



1-00 v 



5 



1-71 „ 



•85 „ 



8 



1-91 „ 



•76 „ 



10 



2-01 „ 



•73 „ 



12 



2-29 „ 



•64 „ 



The velocity of the a particle is thus reduced only 36 per 

 cent, after passing through 12 layers of aluminium. 



Since the a. particle produces approximately the same 

 number of ions per cm. of path in air over its whole range, 

 the simplest assumption to make is that the energy of the 

 a particle is diminished by a constant amount in passing- 

 through each layer of aluminium-foil. After passing through 

 12 layers, the kinetic energy of the a particle is reduced to 

 41 per cent, of the maximum. Each layer of foil thus 

 absorbs about 4' 9 per cent, of the maximum energy. The 

 observed kinetic energy of the a particle after passing- 

 through different thicknesses of aluminium-foil, and the value 



