167 



The results of a similar set of experiments without a 

 Pb dome and with Zn as radiator are shown in fig. iii., 

 curve A . The maximum value of the effect measured is con- 

 siderably greater than in the case of Pb and occurs at a 

 thickness of about 2 cm. of the radiator; for increased thick- 



nesses the curve falls as in the case of Pb. It will be noticed 

 that with sufficient thickness of radiator in this case the 

 curve falls below zero. This was found to be due to the 

 fact that a considerable amount of secondary y radiation, 

 proceeding from the sides of the conical hole in the Pb block 

 containing the Ra, could enter the ionization chamber as 



