[weight] 



VARIATIONS IN THE CONDUCTIVITY OF AIR 



97 



these results it is clear that for all positions of the radium a secondary 

 radiation of considerable intensit}^ 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. 



Radius of circle on which radium was placed, about 1 metre. 



Distance of radium from wall (cms). 



Ionisation (arbitrary scale). 



5 . 5 cms. 

 10.0 



72.93 

 71.74 



In order to obtain further information regarding this secondary 

 radiation the instrument provided with an aluminium cylinder was 

 moved to one of the comers of the room and a similar set of observ- 

 ations made, the values obtained for '' n " in this case being given in 

 Table VIII, and illustrated by the curves in Fig. 7. 



TABLE VIIT. 



Distance of aluminium receiver from far wall = 300 cms. 

 Distance of aluminium receiver from near wall = 105.5 cms. 

 Potential of receiver = 250 volts positive. 

 Radium placed on ciicle 100 cms. radius, with receiver at centre. 



Sec. III., 1908. 7. 



