THE 



AMERICANJOURNALOFSCIENCE 



[FOURTH SERIES.] 



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

 the Influence of Alpha Bays ; by H. A. Bumstead and A. 

 G-. McGougan. 



Introduction. 



In a previous paper under the same title by one of the pres- 

 ent authors,* an account was given of some experiments upon 

 the so-called S-rays which are emitted by metals when struck 

 by a-rays. The emission is known to consist of electrons mov- 

 ing with comparatively small velocities. Maximum estimates 

 of their velocity, based upon the potential difference necessary 

 to cause saturation of the current carried through a high 



CIY1 



vacuum by these electrons, give about 3' X 10 8 , correspond- 



SGC 



ing to a potential difference of about 25 volts ; minimum esti- 

 mates, obtained by measuring the positive potential which a 

 source of S-rays will attain if insulated in a high vacuum, give 

 velocities corresponding to 1 to 3 volts. Campbellf has 

 recently brought forward some evidence for believing that the 

 electrons have considerably smaller velocities even than this, 

 if indeed they have any measurable velocity at all. The 

 experiments to be described in § 3 of this paper, however, 

 render this conclusion improbable. 



In the previous paper it was shown that there was a close 

 analogy between the emission of S-electrons by a metal and the 

 ionization of a gas by a-rays. The number of electrons 

 emitted by the metal varies with the speed of the a-particles 

 in the same manner as the number of ions produced in a gas ; 

 so that, by interposing various thicknesses of aluminium foil 



*This Journal, xxxii, 403, 1911 ; Phil. Mag., xxii, 907, 1911. 

 f Phil. Mag., xxiii, 481, 1912. 



Am. Jour. Sci.— Fourth Series, Vol. XXXIV, No. 202.— October, 1912. 



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