Electrons by Metals under Influence of Alpha Rays. -171 



abscissae o£ II. from La and represents the S-ray effect, or 

 " ionization "of the metal. 



It will be observed from an inspection of Curve II. that the 

 number of a- particles which reach the electrode apparently 

 decreases as more foils are interposed. Up to five thick foils 

 this decrease is approximately linear, and amounts to about 

 20 per cent, of the total. This is too great a falling off to be 

 attributed to the scattering of the a-rays according to the 

 results obtained by Geiger*. It is possible that an 

 explanation of this effect may be found in the phenomena to 

 be discussed in the following section. Beyond five thick 

 foils, the number of a-particles diminishes rapidly ; this is 

 doubtless due to their absorption in the aluminium foils, the 

 more oblique rays being the first to be stopped. 



By dividing the abscissae of I.b by those of II., a curve 

 could be obtained which would represent the ionization or 

 8-ray effect produced by a fixed number of a-particles, which 

 is, strictly speaking, what should be given by an ionization 

 curve. Nevertheless it does not seem advisable to use results 

 thus obtained in the present investigation ; the a-ray currents 

 are small and the errors introduced by using them as factors 

 might be considerable. Moreover, our purpose is primarily 

 to compare the effects with different metals. The ionization 

 curves of gases and vapours have also usually been obtained 

 without allowance for the decrease in the number of a- 

 particles, so that a more direct comparison with them is 

 possible by using the curve l.b. An estimate of the number 

 of S-electrons due to one <x-particle may be obtained, however, 

 by dividing the abscissae of I.b by those of II. Allowing for 

 the fact that the charge on an a-particle is twice the electronic 

 charge, we find that the number of S-electrons per a-particle 

 emitted by an aluminium plate from the incidence side only 

 varies from 7 to 17 as the speed of the a-rays is gradually 

 reduced. 



One very striking result appeared in the course of these 

 experiments which was quite unexpected from anything 

 previously known as to the effects of a- rays. As the number 

 of aluminium foils is decreased, the ionization follows a 

 perfectly regular Bragg curve until only one thin foil is left. 

 When, however, this is removed so that there is no obstacle 

 between the polonium and the electrode, a very large increase 

 is observed in the S-ray current. Thus in the series repre- 

 sented by fig. 2, the £-ray current for one thin foil is 7*82 

 while for no foils it is 16*27, an increase of 107 per cent. It 

 has been shown that this is due to a very absorbable radiation 

 * Proc. Eov. Soc. lxxxiii. p. 492 (1910). 

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