Different Types of Beta Hays. 135 



for magnetic fields excited by currents of ten amperes and 

 greater, the /3 rays were all turned aside, and the 7 rays 

 alone were left to enter the chamber. The maximum ionization 

 due to the j3 and (3 secondary, 7 and 7 secondary, and that due 

 to natural causes is given by the ordinate of the initial point 

 of this curve. With the interpretation given above the 

 ionization due to the 7, 7 secondary, and to any radiations 

 from the metal forming the walls of the chamber, may be 

 taken to be represented by the point on the curve corre- 

 sponding to the highest field. The difference between the 

 values of these two ionizations gives a value for the maximum 

 conductivity impressed upon the air by the ft ravs, and by 

 the secondary radiations excited by them in the tinfoil. 





Table Y. 







Max. 8+fi 





Thickness of Tinfoil 



secondary y + y secon- 



Max. /3+/3 



over bottom of 



+ y + y secon-dary-f- natural 



secondary 



chamber. 



dary 4- natural ionization, 

 ionization. 



ionization. 



mm. 







00000 



1086 51-8 



1034-2 



0-0081 



1121 475 



1073-5 



0-0196 



1165 44-7 



11163 



0-039 



1198 44-7 



11533 



00784 



1225 426 



1182-4 



0-1568 



1230 432 



1186-8 



06272 



1234 43-2 



1190-8 



1-96 



1230 41-5 

 i 



1188-5 



In Table V. is given the deduced values of the maximum 

 ionizations which were due to /3 and /3 secondary rays from 

 similar sets of measurements for different thicknesses of tin- 

 foil at the bottom of the chamber. The curve drawn in 

 fig. 8 is plotted with ordinates representing the values of 

 these maximum /3 and /3 secondary ionizations, as recorded 

 in the fourth column of this table, and with abscissse repre- 

 senting the corresponding thicknesses of tinfoil. From this 

 curve it is clear that the maximum conductivities produced 

 by the /3 and the reflected /3 secondary rays reached a limiting 

 value when the tinfoil sheets attained a thickness of 024 mm., 

 and for still greater thicknesses remained constant. 



Summarizing all the results obtained with tinfoil it would 

 then appear: — That when /3 rays from radium are allowed 

 to impinge on sheets of tinfoil, a maximum reflected secondary 

 radiation is obtained when the tinfoil attains a thickness of 

 024 mm.; and further, that a thickness of 2'5 cams, of tinfoil 



