Conductivity of Air enclosed in Metallic Receivers. 307 



increased, with indications of a possible smaller maximum 

 for a definite angle subtended at the wall by the line joining 

 the radium with the electroscope. A curve representing the 

 variation of " n " with the distance of the radium from the 

 wall is given in fig. 6 (PI. V.), From these results it is clear 

 that for all positions of the radium a secondary radiation of 

 considerable intensity was emitted by the brick wall under 

 the excitation of the rays from the radium. 



Table VII. 



Cylinder of zinc. 



Potential of cylinder =250 volts, positive. 



Kadius of circle on which radium was placed about 1 metre. 



Distance of radium 

 from wall (cms.). 



Ionization 

 (arbitrary scale). 



d'b cms. 



72-93 



100 



71-74 



16-2 



6916 



23-4 



6983 



28-6 



69-16 



636 



67-94 



98-0 



66-31 



1460 



65-70 



55 



72-93 



In order to obtain further information regarding this 

 secondary radiation the instrument, provided with an 

 aluminium cylinder, was moved to one of the corners of the 

 room, and a similar set of observations made, the values 

 obtained for " n " in this case being given in Table VIII. 

 (p. 308) and illustrated by the curve in fig. 7 (PL V.). 



Here again it is seen that the effect of the secondarv 

 radiation was quite marked. As the results show, the ioniza- 

 tion steadily decreased to a minimum value, which corre- 

 sponded approximately to the position in which the radium 

 was on the line joining the electroscope to the corner of the 

 room. After passing through this minimum value the ioni- 

 zation then steadily increased, and reached a maximum when 

 the radium was slightly beyond a line drawn from the cylinder 

 perpendicular to the far wall. After this the ionization fell 

 away again as the distance of the radium from both walls 

 was increased. 



The maximum variation in the values of " n " for these 



