144 



S. E. PIERCE. 



metres. Diminishing the thickness of the linings, which 

 was originally about 0*3 centimetres, had no effect on the 

 values of Ted, so the thickness was sufficient in all cases to 

 give the true value. 



In the following table the relative values of kd for the 

 minimum and maximum screenings are compared with 

 those given by Professor Bragg. 





led,. 







At. 

 mass. 





Thickness of 



Thickness of 



Thick- 



M 



Substance. 



P 



screen 



0*47 cm. 



screen 



1 57 cm. 



ness of 

 screen 



P 









Bragg. 





Bragg. 





5*4 cm. 





Lead 



207-1 



11-37 



100 



100 



100 



100 



100 



8*8 



Tin 



1190 



7-29 



58 



58 



68 



63 



68 



9-3 



Zinc 



654 



71 



47 



47-5 



55 



553 



59-5 



8-4 



Copper 



636 



8-93 





46 





52 



59 



6-7 



Iron 



55-9 



7-86 



45 



45 



54 



51 



57 



73 



Aluminium 



27-1 



2-65 



40 



41-5 



49 



47-5 



54 



20-4 



Card 



12± 



0-9 ± 



39 



395 



46 



45 



52 



50+ 



In the last column of the above table are given the 

 values of kd for the maximum screening divided by the 

 density of the material. If it is assumed that k is the 

 same for the different metals, the numbers represent the 

 relative lengths of path of the fi rays in the various 

 substances. 



