Metals under Influence of Alpha Rays. 409 



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

 stants. In the apparatus described, Bp was equal to A when 

 p=-0G8 mm , so that even with a pressure of several thousandths 

 of a millimeter, the gas effect was small in comparison 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 corre- 

 sponding to a distance in air from the polonium of 2'07 cm . At 

 this point in the range Geiger'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 



45 X -068 



4000 £ = 16. 



760 



Half of these (the positives) readied the electrode, and gave an 

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

 ber of electrons leaving both sides of the foil for each a-parti- 

 cle 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 emis- 

 sion 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 , 64xl0 -lcm in thickness and its approximate 

 air-equivalent, according to Taylor's results, was - 116 cm ; the 

 other was 32 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 rapidly decreased. 

 Curve I, fig. 2, shows the result of a series of such measure- 

 ments made on Jan. 12, 1911. In the figure, the currents in 

 volts per minute are plotted as abscissae, and the number of 

 layers of foil as ordinates ; in order to facilitate comparison 

 with the ionization curve of polonium, however, the scale of 

 ordinates indicates the air-equivalents of the foils. Each plot- 



* Proc. R. S., lxxxii, 486, 1909. 



f Hauser, Phys. Zeits., xii, 466, 1911, whose paper appeared after trie 

 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 .. 



