318 Bumstead and McGougan — Emission of Electrons by 



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 pre- 

 viously 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 ob- 

 served in the 8-ray current. Thus in the series represented by 

 fig. 2, the S-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 consisting 

 partly of electrons moving with considerably higher velocities 

 than the hitherto recognized S-rays. An investigation of this 

 absorbable radiation will be described in the next section. 



Experiments similar to those which have been described at 

 length in the case of aluminium were made also with copper, 

 gold, lead, and platinum. In all cases the surfaces of the 

 metals were made clean and bright by fine sandpaper. The 

 magnitude of the S-ray currents obtained from the various 

 metals under similar conditions were not very different from 

 each other, when correction was made for the decay of the 

 polonium in the intervals between the experiments. The main 

 purpose of the present investigation was to ascertain the varia- 

 tions in the form of the ionization curves for different metals 

 and not their absolute magnitudes ; for this reason no attempt 

 was made to get an accurate determination as to the latter 

 point. It is rendered difficult by the fact, discussed in §1, that 

 the S-ray current falls off with the lapse of time after the 

 liquid air has been applied to the charcoal. This effect was 

 observed in all the metals studied, but it was not quite so 

 marked as in the case of the thin platinum foil described 

 in §1. However, the diminution in the current sometimes 

 amounted to as much as 20 per cent and continued to be 

 noticeable for two or three days.* As in the case of the 

 platinum foil, the relative values at different points of the 

 range of the a-particles were not affected by this variation, the 

 ratio of the ionizations at any two points remaining practically 

 constant. 



The results for the different metals are given in Table III 

 and plotted in fig. 3. The values used are the currents due to 

 the 8-electrons alone, corresponding to Curve lb in fig. 2. In 

 order to make the comparison easier they have been reduced to 

 the same scale by making the current for one thick foil the 



* Our observations in regard to the variations in the magnitude of the 

 <J-ray current with different metals is in substantial agreement with those of 

 Campbell, Phil. Mag., xxi, 298, 1911. 



