Electrons by Metals under Influence of Alpha Rays. 463 



metals when struck by a-rays. The emission is known 

 to consist of electrons moving 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 vacuum by these electrons, 



cm 

 give about 3 x 10 8 : . corresponding to a potential differ- 

 ence of about 25 volts ; minimum estimates, obtained by 

 measuring the positive potential which a source of S-rays 

 will attain if insulated in a high vacuum, give velocities cor- 

 responding to 1 to 3 volts. Campbell * 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 between the source of a-rays and the metal, one can plot 

 a curve entirely similar to the ionization curves first obtained 

 by Bragg. The number of electrons emitted by the metal 

 increases with the number of foils interposed until their 

 combined thickness is nearly equal to the range of the a-rays 

 in aluminium, after which the emission of electrons falls off 

 rapidly. The increase in the number of electrons with 

 diminishing speed of the a-rays, however, is not so great as 

 the increase in the number of ions in a gasT. The ionization 

 curve of the metal lies within, or to the left of, the corre- 

 sponding curve for gaseous ionization and has a less pro- 

 nounced maximum or " knee " just before the end of the range 

 is reached. This result was anticipated before the experi- 

 ments were undertaken, for reasons given in the previous 

 paper. The same arguments which led to this conclusion 

 also gave reason to believe that the curve for a metal of high 

 atomic weight, such as gold, would lie within that of a metal 

 of lower atomic weight such as aluminium, just as the latter 

 would lie within the curve of gaseous ionization. This 

 expectation, however, was not fulfilled by the results of 

 the experiments. The curves for gold and for aluminium 



* Phil. Mag. xxiii. p. 481 (1912). 



t The increase in the emission as the speed of the a-particles decreases 

 has also been observed by Campbell, Phil. Mag-, xxi. p. 276 (1911). 



