hy Metals wider the Influence of Alpha Hays. 913 



where i is the current, p the pressure, and A and B are 

 constants. In the apparatus described, Bp was equal to A 

 when p = *068 mm., so that even with a pressure of several 

 thousandths of a millimetre, the gas effect was small in com- 

 parison with the metal effect. 



The above measurements (which were incidental and some- 

 what rough) permit an estimate of the number of electrons 

 emitted when one a-particle passes through the aluminium 

 foil electrode. The a-rays were at a point in their range 

 corresponding to a distance in air from the polonium of 2-()7 

 cm. At this point in the range Greiger's * results show that 

 an a-particle produces about 4000 ions per mm. of its path 

 in air at standard pressure. In the present case, the depth 

 of the ionization-chamber was 40 mm. ; most of the rays 

 passed through somewhat obliquely and the average distance 

 traversed was about 45 mm. At a pressure of *068 mm. the 

 ions per a-particle would be 



4000^^=16. 



Half of these (the positives) reached the electrode and gave 

 an effect equal to that of the electrons which left it. The 

 number of electrons leaving both sides of the foil for each 

 a-particle passing through it is thus approximately eight f . 



The foregoing preliminary experiments having shown that 

 the effects observed in the electroscope were due to the 

 emission of electrons from the surfaces of the metal, the 

 speed of the a-particles was varied by interposing sheets of 

 aluminium foil between the polonium and the cover of the 

 exhausted chamber. Foils of two different thicknesses were 

 used; the thinner was 0*64 x 10"' 1 cm. in thickness and its 

 approximate air-equivalent, according to Taylor's results, 

 was 0'116 cm. ; the other was 3"2 x 10~ 4 cm. thick and was 

 equivalent to 0'58 cm. of air ; it happened to be just five 

 times as thick as the first foil. 



With the case charged to + 40 volts, the current of 

 electrons from the electrode was measured as successive 

 layers of foil were interposed. The current increased until 

 five or six of the thin foils had been added, after which it 



* Proc. Roy. Soc. lxxxii. p. 486 (1909). 



t Hauser, Fhys. Zeitsch. xii. p. 4G6 (1911), whose paper appeared after 

 the completion of these experiments, finds that about 20 electrons are 

 emitted from one side (the emergence side) of an aluminium foil 

 for each a-particle. 



