Metals under Influence of Alpha Rays. 



415 



In order to test the effect with another metal, three gold 

 leaves were put upon the rings, E, F and F', instead of alumin- 

 ium. These leaves were *09 X lO"" " 1 thick and had an air 

 equivalent of approximately 0-05™ (about half of that of the 

 aluminium leaf). The emission from the gold electrode was 

 about 13 per cent less than from the aluminium but the varia- 

 tion with the speed of the a-rays was very nearly the same. 

 In the following table are given the averages for all the exper- 

 iments with both metals (with positive potential on the case); 

 the currents with two foils are called 100 as before. Included in 

 the table are two series of ionization measurements. These 

 were made by removing the foil from the electrode, E, admit- 

 ting a small quantity of air (pressure - 032 mm ), and charging the 

 case positively. The first of the two ionization experiments is 

 with the aluminium-leaf on F and F', the second with the gold- 

 leaf. 



Table II. 



No. Foils 



2 



3 



4 



5 



6 



Al 



100 

 100 

 100 

 100 



105-2 

 1056 

 110-5 



no- 



117-6 



118-2 



134- 



130- 



107-3 

 115-6 

 US- 



39-1 



Gold 



Air(l) 



Air (2)... 



72-2 

 57-3 

 96-5 



As will be seen, the course of the series with the two 

 metals is nearly identical ; the differences in the last column 

 are to be attributed to the greater stopping power of the 

 aluminium leaf; it appears also in the two ionization experi- 



a small thickness of aluminium was in the path of the rays, the magnetic 

 field increased this positive charge, but only slightly ; (evidently the number 

 of electrons emitted by the window and falling upon the plate was in excess 

 of the number emitted by the plate itself, but this excess was not enough to 

 neutralize the charge carried by the a-rays.) When the a-rays had to pass 

 through a thicker layer of aluminium, the application of the magnetic field 

 diminished the positive charge acquired by the plate ; (in this case, the elec- 

 trons emitted by the plate appear to be in excess of those received from the 

 window.) The author draws the conclusion that "with diminishing speed 

 of the primary a-particles, it appears that the intensity of the secondary 

 radiation increases, at first slowly, and finally considerably." It does not 

 appear that the conclusion is altogether justified by the experiment ; the 6- 

 radiation from the window seems to have been left out of consideration ; the 

 most that can be concluded is that there is a differential effect as between 

 copper and aluminium 



Another effect which is doubtless due to the same cause was observed 

 by Duane (C. E., cxlvi, 1088, 1908). He found that the secondary radiation 

 ceases a little less abruptly at the end of the range than the charge on the 

 a-particles. As he used an unlimited beam of rays, this result may woll have 

 been due to the increased emission of electrons near the end of the range. 



