176 



To Earth ^ 



To Cells ^ 



555^ 



^^^^m4i^^^^ 



mf'^m if S^f^ 



20cms ^ FIG, VII. 



The radiators shown by horizontal dotted lines below p,p 

 intercept the radiation from the Ra. By means of Pb plates L^L 

 an amount of the secondary radiation may be prevented from 

 entering the ionization chamber. The dotted conical chamber 

 was used in a separate set of experiments. 



allowing the y rays to pass without obstruction to the ioni- 

 zation chamber ; while a considerable amount of secondary y 

 radiation from the radiating plates p,p was prevented from 

 reaching the chamber. A strong magnetic field was applied 

 during the experiments. The floor of the ionization chamber, con- 

 sisting of Pb 1*25 mm. thick, was sufficient, as has been 

 shown in previous papers, to give the full amount of emer- 

 gence /8 radiation. Several readings were taken with each 

 thickness of radiator in position, and the readings were suf- 

 ficiently large with the chamber filled with air. From any 

 reading it is necessary to subtract an amount corresponding 



