220 



Mr. W. Wilson on the Effect of Pressure on 



The ionization was very weak, being only '25 scale- 

 division per minute at atmospheric pressure. This corresponds 

 to about 216 ions per c.c. per second. Such a low ionization 

 was used in order that the comparatively low voltage applied 

 would be sufficient to give the saturation current, without 

 which o£ course the results would have no definite meaning. 

 Some experiments on this subject have been already done 

 by Erikson *, but he makes no statement as to whether he 

 was using the saturation current or not. It was probably 

 not in fact, for with an E.M.F. of 19 volts per cm. he 

 obtained a maximum current at 70 atmospheres pressure, 

 while with 1000 volts the maximum occurred at 140 atmo- 

 spheres. 



The results obtained are given in fig. 3, where the ionization 

 is plotted against the pressure. It will be observed that the 



Fig. 3. 



c 

 P 



J 1 



























































































































Pncssunc 



w Ar/*ost 



hskcs. 



10 



20 



30 



40 



ionization does not increase proportionally with the pressure, 

 but less rapidly as the pressure rises. This confirms the fact, 

 as McLennan has shown, that the secondary rays from the 

 sides of a vessel exposed to 7 rays contribute an appreciable 

 amount to the ionization inside. Thus, since this secondary 

 radiation is comparatively easily absorbed, the ionization 

 due to it will approach a maximum value as the pressure 

 rises, while that due to the 7 rays alone will not do so, but 

 will increase proportionally with the pressure, i. e. with the 

 density of the gas. 



An approximate equation to the curve can be obtained as 

 follows: — Consider for simplicity of calculation that the 

 secondary rays move perpendicular to the sides of the 



* Erikson, Phys. Rev. Jan. 1908. 



