Maximum Frequency of X Rays from a Coolidge Tube. 347 



determined, and the readings corrected for change of 

 capacity of the systems and for inequalities in the electro- 

 meter scale. 



In the later experiments, an additional method was used 

 in order to correct for changes in intensity of the radiation. 

 This depended on the observed fact that the intensity of the 

 radiation for a given voltage was directly proportional to 

 the current through the bulb. The value of the latter was 

 read by means of the galvanometer during the course of 

 each observation by the methods already described. The 

 values obtained by the two standardizing methods were in 

 good agreement, and each served as a useful check on the 

 other. 



Experimental Results. 



Some of the absorption curves in aluminium of the radia- 

 tion at different voltages are shown in fig. 3, where the 



Fig. 3. 



15 20 25 



Thickness of Aluminium in rrms. 



logarithm of the current is ordinate, and the abscissa? the 

 thickness of aluminium. If the absorption were exponential, 

 the curve should be a straight lino, but it is seen that this is 

 not the case for any of the curves. The radiation at first 

 rapidly diminishes owing to the absorption of the softer 

 radiations, and on the average gradually becomes more 



