THE 



AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES.] 



Art. XLIII. — On the Emission of Electrons by Metals under 

 the Influence of Alpha Rays ; by H. A. Bumstead. 



It was observed by Bragg and Kleeman* that when a thin 

 sheet of a metal, such as gold, was interposed in the path of a 

 pencil of a-rays, the diminution in range produced was less 

 when the metal sheet was far from the source of rays than 

 when it was near ; in other words, the rays seemed to be 

 "stopped" less by the metal when they were going slowly 

 than when they were moving with greater speed. T. S. Taylorf 

 made a careful study of the phenomenon and found that this 

 differential effect increased regularly with the atomic weight 

 of the metal. Substances whose atomic weight was much 

 greater than that of air showed the effect very markedly ; 

 substances, such as paper, whose average atomic weight was 

 nearly equal to that of air did not show the effect at all ; and 

 when a layer of hydrogen was used instead of a solid obstacle, 

 the effect was reversed. By plotting the ionization curves 

 of the a-rays in air and in hydrogen, Taylor showed that 

 there was exactly the -same relation between the ionization in 

 the two gases at different parts of the range as between their 

 relative stopping powers. Where the ionization in hydrogen 

 was greater than in air, the hydrogen was more effective in 

 retarding the rays, and vice versa ; and the numerical ratios for 

 the two effects were equal. These results led naturally to the 

 hypothesis that the energy lost by the a-particles in their prog- 

 ress through a gas was mainly, if not wholly, consumed in the 

 production of ions, and that where the ionization was great (as 

 at the " knee " of the Bragg curve), the a-particles lost energy 



*Phil. Mag., x, 318 (1905). 



f This Journal, xxviii, 357 (1909). Phil. Mag., xviii, 604, 1909. 



Am. Jour. Sci. — Fourth Series, Vol. XXXII, No. 192. — December, 1911. 

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