MEASUREMENT OF X-RAYS AND RADIUM 53 



Below 100 kv. (constant), absorption in aluminum has been recom- 

 mended as a standard, hence Table 3 gives alimiinum absorption data. 

 In makmg these measurements the same thin glass tube as for Table 1 

 was used. 



Figures 5, 6, and 7 give, respectively, for various constant-potential 

 excitations, the logarithmic absorption curves for copper and aluminum 

 as plotted from Tables 1, 2, and 3, respectively (51). If it is assumed 

 that, tor the same excitation potential, the spectral distribution of the 

 radiation emitted by the anticathodes of the two tubes is the same, then 

 the difference between the copper absorption curves corresponding to 

 the same potential in Figs. 5 and 6 arises entirely from the difference 

 in the absorption of the two tube walls-1.3 mm. cerium glass, and 4 8 

 mm. pyrex glass. 



