482 Emission of Electrons under Influence of Alpha Rays. 



ns — 0"14 ; the ratio of these currents is 0*11. Now the 

 plate subtends at the foil a solid angle of 47rx015* : for 

 an electron generated in the plate itself we may take the 

 solid angle as 4t. Taking the figures as thev stand, this 

 would indicate that at least \\ of the supposed a-ray effect 

 is produced indirectly by means of the secondary electrons 

 generated in the plate. We may suppose that they give rise 

 to the slow electrons, either as they emerge from the plate 

 itself, or as they pass through the layer of gas which we 

 have been led to postulate by the results of the preceding- 

 section. 



If this should prove to be the mechanism of the emission 

 of 8-rays by metals, it will at least render it probable that 

 a similar process takes place when gases are ionized by 

 a-rays. The column of ions following the track of the 

 a-particle, which was assumed by Moulin, and which has 

 recently been made visible in the beautiful experiments of 

 0. T. K. Wilson, would then be made up of the tracks of 

 many secondary electrons, radiating irregularly from the 

 axis of the column, and extending only a small fraction of a 

 millimetre from it. 



Summary. 



1. The number of S-electrons emitted by a metal when 

 struck by a-rays varies with the speed of the a-rays in the 

 same manner as does the number of ions produced in a gas. 

 Curves which represent this variation are similar in form to 

 the Bragg ionization curve. 



2. Such curves have been determined for aluminium, 

 copper, gold, lead, and platinum. Within the limits of 

 accuracy of the experiments they have the same form for 

 all these metals ; this agreement is in contrast with the fact 

 that ionization curves for different gases and vapours show 

 marked differences in form. 



3. The magnitude of the S-ray effect, and the field 

 necessary to cause saturation, decrease progressively for some 

 time after the vacuum is produced. This is ascribed to the 

 gradual removal of a layer of adsorbed gas ; during this 

 process the form of the "ionization curve " does not change. 

 This suggests that the whole effect may be due to such a layer 

 which cannot be entirely removed; the similarity of the curves 

 obtained with different metals would thus find a natural 

 explanation. However, an experiment, in which a strip of 

 platinum was heated to red heat in a high vacuum, failed 

 to o-ive evidence of the removal of this assumed film. 



* The dimensions are different from those shown in fig-. 1. 



