136 Prof. E. Rutherford on Uranium Radiation and 



very different, and would vary greatly with the voltage and 

 distance between the electrodes as well as with the gas. It 

 has not been considered necessary to introduce the results 

 obtained for small voltages in this paper, as they are very 

 variable under varying conditions. Although they may all be 

 simply explained on the results obtained for the saturating 

 electromotive forces they do not admit of simple calculation, 

 and only serve to obscure the simple laws which govern the 

 relations between ionization, absorption, and pressure. The 

 general nature of the results for low voltages can be deduced 

 from a consideration of the results given for the connexion 

 (see § 16) between the current through the gas and the 

 electromotive force acting on it at various pressures. 



The above results for the relation between current and 

 pressure may be compared with those obtained for Rontgen 

 radiation. Perrin * found that the rate of leak varied directly 

 as the pressure for saturating electromotive forces when the 

 radiation did not impinge on the surface of the metal plates. 

 This is in agreement with the results obtained for uranium 

 radiation, for Perrin's result practically asserts that the ioni- 

 zation is proportional to the pressure. The results, however, 

 of other experimenters on the subject are very variable and 

 contradictory, due chiefly to the fact that in some cases the 

 results were obtained for non-saturating electromotive forces, 

 while, in addition, the surface ionization at the electrodes 

 greatly complicated the relation, especially at low pressures. 



§11. Amount of Ionization in Different Gases. 



It has been shown that the oc radiation from uranium is 

 rapidly absorbed by air and other gases. In consequence of 

 this the total amount of ionization produced, when the radia- 

 tion is completely absorbed, can be determined. 



The following arrangement was used : — A brass ball 2*2 cm. 

 in diameter was covered with a thin layer of uranium oxide. 

 A thin brass rod was screwed into it and the sphere was fixed 

 centrally inside a bell-jar of 13 cm. diameter, the brass rod 

 passing through an ebonite stopper. The bell-jar was fixed 

 to a base-plate, and was made air-tight. The inside and out- 

 side of the bell-jar were covered with tinfoil. In practice an 

 E.M.F. of 800 volts was applied to the outside of the bell- 

 jar. The sphere, through the metal rod, was connected to 

 one pair of quadrants of an electrometer. It was assumed 

 that, with such a large potential-difference between the bell- 

 jar and the sphere, the gas was approximately saturated and 



* Comptes Rendus, cxxiii. p. 878. 



