by Metals under the Influence of Alpha Rays. 919 



Table I. 



Date. 



Volts 



on 

 Case. 



Number of Foils. 



2 



3. 



4. 



5. 



6. 



May 23 



May 27 



June 9 



>> 



+ 160 

 -160 

 + 80 



- 80 

 + 80 



- SO 



100 

 100 

 100 

 100 

 100 

 100 



1072 



101 



101-5 



101-6 



106 



101-5 



118-5 



1182 



116-2 



119 



118 



119 



107 



107 



106-8 



103-6 



109 



110 



39 

 54 



38-7 

 55-5 

 39-5 

 51-8 



the nearly horizontal " top " of the curve, and a small 

 difference in the range makes a considerable difference in 

 the effect *. 



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

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

 aluminium. These leaves were *09 x 10 ~ 4 cm. thick and had 

 an air-equivalent of approximately 0*05 cm. (about half of! 

 that of the aluminium leaf). The emission from the gold 



* An effect probably due to this increased emission of electrons with 

 diminishing speed of the a-particles was observed by Aschkinass (Ann. 

 d. Phijs. xxvii. p. 377 (1908)). He was measuring the charge of the a- 

 particles after passing through various thicknesses of aluminium, and had 

 a transverse magnetic field to curl up the d-rays. The a-ravs passed 

 through an aluminium window into a separate chamber and fell upon a 

 copper plate. Even without a magnetic field the plate acquired a positive 

 charge ; when 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-ravs). When the a-rays had to pass through a thicker layer of alum- 

 inium, the application of the magnetic field diminished the positive charge 

 acquired by the plate (in this case, the electrons 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 in- 

 creases, at first slowly, and finally considerably." It does not appear that 

 the conclusion is altogether justified by the experiment; the 8-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. R. clxvi. p. 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 well have been due to the increased emission of electrons near 

 the end of the range. 



3P2 



